name	policyengine-variable-patterns
description	PolicyEngine variable patterns - variable creation, no hard-coding principle, federal/state separation, metadata standards

PolicyEngine Variable Patterns

Essential patterns for creating PolicyEngine variables for government benefit programs.

FIRST PRINCIPLE: Legal Code is the Source of Truth

The law defines WHAT to implement. These patterns are just HOW to implement it.

1. READ the legal code/policy manual FIRST
2. UNDERSTAND what the law actually says
3. IMPLEMENT exactly what the law requires
4. USE these patterns as tools to implement correctly

Patterns are tools, not rules to blindly follow:

If the legal code says something different from common patterns → FOLLOW THE LAW
If another state does it differently → Check YOUR state's legal code
If a pattern doesn't fit the regulation → Implement what the law says

Every implementation decision should trace back to a specific legal citation.

Modeling Time-Limited Rules

PolicyEngine supports modeling time-limited disregards. DO implement these — DO NOT skip them.

Not Yet Modeled

Lifetime benefit limits (e.g., federal 60-month TANF limit) are not yet modeled in PolicyEngine. Don't implement these, but don't add comments claiming they're architecturally impossible — they may be added in the future.

Pattern 1: Calendar Month — Time-Varying Disregard

Use when a disregard rate changes based on the month of the year. Uses period.start.month to get the calendar month (January=1, February=2, etc.).

class state_tanf_countable_earned_income(Variable):
    def formula(spm_unit, period, parameters):
        p = parameters(period).gov.states.xx.tanf.income
        earned = spm_unit("tanf_gross_earned_income", period)

        month = period.start.month
        tlp_rate = p.time_limited_percentage.rate.calc(month)
        return earned * (1 - tlp_rate)

Parameter structure — bracket by month:

# .../time_limited_percentage/rate.yaml
brackets:
  - threshold:
      1997-07-01: 1
    amount:
      1997-07-01: 0.5    # Months 1-6: 50%
  - threshold:
      1997-07-01: 7
    amount:
      1997-07-01: 0.35   # Months 7-9: 35%
  - threshold:
      1997-07-01: 10
    amount:
      1997-07-01: 0.25   # Months 10-12: 25%

Pattern 2: `applicable_months` Split

Use when a state splits the year into periods with different disregard rates.

class state_tanf_countable_earned_income(Variable):
    def formula(spm_unit, period, parameters):
        p = parameters(period).gov.states.xx.tanf.income
        # First 6 months: 100% disregard (if eligible)
        # Remaining 6 months: partial disregard
        full_disregard_months = p.full_disregard.applicable_months  # 6
        remaining_months = MONTHS_IN_YEAR - full_disregard_months
        # ... apply appropriate rate based on period

Critical Principles

1. ZERO Hard-Coded Values

Every numeric value MUST be parameterized

❌ FORBIDDEN:
return where(eligible, 1000, 0)     # Hard-coded 1000
age < 15                             # Hard-coded 15
benefit = income * 0.33              # Hard-coded 0.33
month >= 10 and month <= 3           # Hard-coded months

✅ REQUIRED:
return where(eligible, p.maximum_benefit, 0)
age < p.age_threshold.minor_child
benefit = income * p.benefit_rate
month >= p.season.start_month

Acceptable literals:

0, 1, -1 for basic math
12 for month conversion (/ 12, * 12)
Array indices when structure is known

2. No Placeholder Implementations

Delete the file rather than leave placeholders

❌ NEVER:
def formula(entity, period, parameters):
    # TODO: Implement
    return 75  # Placeholder

✅ ALWAYS:
# Complete implementation or no file at all

3. Use `adds` or `add()` - NEVER Manual Addition

CRITICAL: NEVER manually fetch variables and add them with +. Always use adds or add().

Rule 1: Pure sum → `adds` attribute (no formula)

❌ WRONG - Writing a formula for simple sum:
class tx_tanf_gross_income(Variable):
    def formula(spm_unit, period, parameters):
        earned = spm_unit("tanf_gross_earned_income", period)
        unearned = spm_unit("tanf_gross_unearned_income", period)
        return earned + unearned  # DON'T DO THIS!

✅ CORRECT - Use adds, no formula needed:
class tx_tanf_gross_income(Variable):
    value_type = float
    entity = SPMUnit
    definition_period = MONTH
    adds = ["tanf_gross_earned_income", "tanf_gross_unearned_income"]
    # NO formula method - adds handles it automatically!

Rule 2: Sum + other operations → `add()` function

❌ WRONG - Manual fetching and adding:
def formula(spm_unit, period, parameters):
    earned = spm_unit("tanf_gross_earned_income", period)
    unearned = spm_unit("tanf_gross_unearned_income", period)
    gross = earned + unearned  # DON'T manually add!
    return gross * p.rate

✅ CORRECT - Use add() function:
def formula(spm_unit, period, parameters):
    gross = add(spm_unit, period, ["tanf_gross_earned_income", "tanf_gross_unearned_income"])
    return gross * p.rate

Decision rule:

Is it ONLY a sum? → adds = [...] (no formula)
Sum + other operations? → add() function inside formula

See policyengine-aggregation-skill for detailed patterns.

Variable Gotchas

`age` is a Float

PolicyEngine's age variable is a float, not an integer. Age boundary parameters must use precise values (e.g., 1.5 for 18 months, not 2).

`monthly_age` Returns Years, Not Months

Despite the name, monthly_age returns age in years. It reverses PolicyEngine's auto-division by 12 for annual variables accessed in monthly periods. For month-based comparisons, use person("age", period.this_year) * MONTHS_IN_YEAR.

`defined_for` + `entity.sum() / N` Couple Gotcha

When a formula uses defined_for on person-level eligibility combined with marital_unit.sum(variable) / 2 for couples, the ineligible spouse's variable returns 0 (filtered by defined_for). The sum includes that zero, so division by 2 halves the eligible spouse's benefit.

"Eligible Except for Income" Trap

When a program covers people who "would be eligible for [upstream program] except for income," using the upstream is_xxx_eligible (which includes income tests) as a gate excludes the target population. Create a separate eligibility variable without income tests, or document the gap.

Verify Boundary Operators

When eligibility depends on income vs a threshold, verify whether the regulation says "less than" (<) or "at or below" (<=). The wrong operator silently misclassifies people at exactly the threshold.

Child Status: Use Age, Not Tax Dependency

For benefit eligibility, use an age-based check (age < 18), not is_tax_unit_dependent. Tax-unit dependents include elderly parents and exclude non-dependent minors.

Income Exclusion for Exempt Recipients

When a program exempts recipients of another program (e.g., SSI), their income must also be excluded from countable income. Exemption of a person implies exemption of that person's income.

Parameterize FPL Reference Year

When a standard is defined as a percentage of FPL, parameterize the FPL reference year rather than using the current-period FPL. The pinned year may differ from the benefit period.

Minimum Positive Payment

Check whether the regulation defines a minimum payment for near-threshold recipients (e.g., $1/month). Omitting this floor zeroes out benefits for a narrow but real population.

Document Unmodeled Pathways

When a program has multiple eligibility pathways, document which are modeled and which are excluded in the variable docstring.

`is_ssi_eligible` Does NOT Include an Income Test

is_ssi_eligible only checks aged/blind/disabled status, resource test, and immigration status. Income is handled separately via uncapped_ssi in benefit formulas. To check if someone actually receives SSI, use uncapped_ssi > 0 (positive means income is below the SSI benefit amount). State SSP implementations must check both is_ssi_eligible AND uncapped_ssi > 0 — SSP requires being an SSI recipient, not just categorically eligible.

# ❌ WRONG — includes people with too much income:
eligible = person("is_ssi_eligible", period)

# ✅ CORRECT — actual SSI recipients only:
categorically_eligible = person("is_ssi_eligible", period)
receives_ssi = person("uncapped_ssi", period) > 0
eligible = categorically_eligible & receives_ssi

Variable Implementation Standards

Variable Metadata Format

Follow established patterns:

class il_tanf_countable_earned_income(Variable):
    value_type = float
    entity = SPMUnit
    definition_period = MONTH
    label = "Illinois TANF countable earned income"
    unit = USD
    reference = "https://www.law.cornell.edu/regulations/illinois/..."
    defined_for = StateCode.IL

    # Use adds for simple sums
    adds = ["il_tanf_earned_income_after_disregard"]

Key rules:

✅ Use full URL in reference (clickable)
✅ For PDF links, include page number: #page=XX
✅ For multiple references, use TUPLE () not list []
❌ Don't use documentation field - use reference instead
❌ Don't use statute citations without URLs

❌ WRONG - Don't use documentation field:

class some_variable(Variable):
    documentation = "This is the wrong field"  # DON'T USE THIS

✅ CORRECT - Use reference field:

class some_variable(Variable):
    reference = "https://example.gov/rules.pdf#page=10"  # USE THIS

Scoping `defined_for` to the Right Level

defined_for controls which entities run the formula. Use the most specific scope that fits:

# ❌ TOO BROAD — calculates rates for all RI residents (adults, ineligible children)
class ri_ccap_licensed_center_rate(Variable):
    entity = Person
    defined_for = StateCode.RI

# ✅ CORRECT — only calculates rates for children eligible for CCAP
class ri_ccap_licensed_center_rate(Variable):
    entity = Person
    defined_for = "ri_ccap_eligible_child"

Guidelines:

SPMUnit-level benefit variables → defined_for = "state_program_eligible" (the main eligibility variable)
Person-level variables within a program (rates, per-child amounts) → defined_for = "state_program_eligible_child" or the person-level eligibility variable
Eligibility check variables themselves → defined_for = StateCode.XX (they determine eligibility, so they can't depend on it)

This avoids unnecessary computation and makes the variable's scope clear from its definition.

Reference format:

# Single reference:
reference = "https://oregon.gov/dhs/tanf-manual.pdf#page=23"

# Multiple references - use TUPLE ():
reference = (
    "https://oregon.public.law/rules/oar_461-155-0030",
    "https://oregon.gov/dhs/tanf-manual.pdf#page=23",
)

# ❌ WRONG - Don't use list []:
reference = [
    "https://...",
    "https://...",
]

When to Use `adds` vs `formula`

CRITICAL: Never use both adds/subtracts AND a custom formula in the same variable!

This causes bugs when the two get out of sync. Choose one approach:

❌ FORBIDDEN - Mixing compositional and formula:
class household_net_income(Variable):
    subtracts = ["employee_pension_contributions"]  # ❌ Has subtracts

    def formula(household, period):  # ❌ AND has formula
        gross = household("household_gross_income", period)
        tax = household("income_tax", period)
        # BUG: Forgot to subtract employee_pension_contributions!
        return gross - tax

Use adds/subtracts when:

Just summing variables
Passing through a single variable
No transformations needed

✅ BEST - Pure compositional:
class tanf_gross_income(Variable):
    adds = ["employment_income", "self_employment_income"]

✅ BEST - Compositional with subtracts:
class household_net_income(Variable):
    adds = ["household_gross_income"]
    subtracts = ["income_tax", "employee_pension_contributions"]

Use formula when:

Applying transformations
Conditional logic
Calculations needed

✅ CORRECT - Pure formula:
def formula(entity, period, parameters):
    income = add(entity, period, ["income1", "income2"])
    return max_(0, income)  # Need max_

TANF Countable Income Pattern

Critical: Verify Calculation Order from Legal Code

MOST IMPORTANT: Always check the state's legal code or policy manual for the exact calculation order. The pattern below is typical but not universal.

The Typical Pattern:

Apply deductions/disregards to earned income only
Use max_() to prevent negative earned income
Add unearned income (which typically has no deductions)

This pattern is based on how MOST TANF programs work, but you MUST verify with the specific state's legal code.

❌ WRONG - Applying deductions to total income

def formula(spm_unit, period, parameters):
    gross_earned = spm_unit("tanf_gross_earned_income", period)
    unearned = spm_unit("tanf_gross_unearned_income", period)
    deductions = spm_unit("tanf_earned_income_deductions", period)

    # ❌ WRONG: Deductions applied to total income
    total_income = gross_earned + unearned
    countable = total_income - deductions

    return max_(countable, 0)

Why this is wrong:

Deductions should ONLY reduce earned income
Unearned income (SSI, child support, etc.) is not subject to work expense deductions
This incorrectly reduces unearned income when earned income is low

Example error:

Earned: $100, Unearned: $500, Deductions: $200
Wrong result: max_($100 + $500 - $200, 0) = $400 (reduces unearned!)
Correct result: max_($100 - $200, 0) + $500 = $500

✅ CORRECT - Apply deductions to earned only, then add unearned

def formula(spm_unit, period, parameters):
    gross_earned = spm_unit("tanf_gross_earned_income", period)
    unearned = spm_unit("tanf_gross_unearned_income", period)
    deductions = spm_unit("tanf_earned_income_deductions", period)

    # ✅ CORRECT: Deductions applied to earned only, then add unearned
    return max_(gross_earned - deductions, 0) + unearned

Pattern Variations

With multiple deduction steps:

def formula(spm_unit, period, parameters):
    p = parameters(period).gov.states.xx.tanf.income
    gross_earned = spm_unit("tanf_gross_earned_income", period)
    unearned = spm_unit("tanf_gross_unearned_income", period)

    # Step 1: Apply work expense deduction
    work_expense = min_(gross_earned * p.work_expense_rate, p.work_expense_max)
    after_work_expense = max_(gross_earned - work_expense, 0)

    # Step 2: Apply earnings disregard
    earnings_disregard = after_work_expense * p.disregard_rate
    countable_earned = max_(after_work_expense - earnings_disregard, 0)

    # Step 3: Add unearned (no deductions applied)
    return countable_earned + unearned

With disregard percentage (simplified):

def formula(spm_unit, period, parameters):
    p = parameters(period).gov.states.xx.tanf.income
    gross_earned = spm_unit("tanf_gross_earned_income", period)
    unearned = spm_unit("tanf_gross_unearned_income", period)

    # Apply disregard to earned (keep 33% = disregard 67%)
    countable_earned = gross_earned * (1 - p.earned_disregard_rate)

    return max_(countable_earned, 0) + unearned

When Unearned Income HAS Deductions

Some states DO have unearned income deductions (rare). Handle separately:

def formula(spm_unit, period, parameters):
    gross_earned = spm_unit("tanf_gross_earned_income", period)
    gross_unearned = spm_unit("tanf_gross_unearned_income", period)
    earned_deductions = spm_unit("tanf_earned_income_deductions", period)
    unearned_deductions = spm_unit("tanf_unearned_income_deductions", period)

    # Apply each type of deduction to its respective income type
    countable_earned = max_(gross_earned - earned_deductions, 0)
    countable_unearned = max_(gross_unearned - unearned_deductions, 0)

    return countable_earned + countable_unearned

Quick Reference

Standard TANF pattern:

Countable Income = max_(Earned - Earned Deductions, 0) + Unearned

NOT:

❌ max_(Earned + Unearned - Deductions, 0)
❌ max_(Earned - Deductions + Unearned, 0)  # Can go negative

Federal/State Separation

Federal Parameters

Location: /parameters/gov/{agency}/

Base formulas and methodologies
National standards
Required elements

State Parameters

Location: /parameters/gov/states/{state}/

State-specific thresholds
Implementation choices
Scale factors

# Federal: parameters/gov/hhs/fpg/base.yaml
first_person: 14_580

# State: parameters/gov/states/ca/scale_factor.yaml
fpg_multiplier: 2.0  # 200% of FPG

State Income Tax Conformity to Federal Rules

CRITICAL: State income taxes should reference federal income sources and limits, not redefine them

Most state income taxes start with federal definitions and then make specific adjustments. When implementing state income tax:

✅ CORRECT - Reference federal income sources:

class ms_agi(Variable):
    """Mississippi adjusted gross income"""
    value_type = float
    entity = TaxUnit
    definition_period = YEAR
    label = "Mississippi adjusted gross income"
    unit = USD

    def formula(tax_unit, period, parameters):
        # Start with federal AGI, which already includes
        # federal capital loss limits and other federal rules
        federal_agi = tax_unit("adjusted_gross_income", period)

        # Apply Mississippi-specific additions/subtractions
        ms_additions = tax_unit("ms_additions_to_agi", period)
        ms_subtractions = tax_unit("ms_subtractions_from_agi", period)

        return federal_agi + ms_additions - ms_subtractions

❌ WRONG - Redefining income sources:

# DON'T create state-specific parameters like:
# parameters/gov/states/ms/tax/income/income_sources.yaml
# containing:
#   - capital_gains
#   - long_term_capital_gains
#   - short_term_capital_gains

# This bypasses federal limits like the $3,000 capital loss deduction limit

Why this matters:

Federal income tax applies capital loss limits before reporting AGI
State income taxes that start from federal AGI automatically inherit these limits
Creating separate state income source parameters bypasses federal rules
Results in incorrect calculations (e.g., unlimited capital loss deductions)

Common state conformity patterns:

Full conformity - State AGI = Federal AGI (rare)
Rolling conformity - State follows current federal rules
Static conformity - State follows federal rules as of a specific date
Selective conformity - State follows federal but with specific modifications

Implementation approach:

Always start with federal income sources/AGI/taxable income as the base
Use state parameters only for state-specific additions, subtractions, or modifications
Reference federal variables: adjusted_gross_income, taxable_income, etc.
Don't recreate federal income aggregation logic at the state level

Example - Mississippi specifics:

class ms_additions_to_agi(Variable):
    """Mississippi additions to federal AGI"""
    # Add state-specific income items not in federal AGI
    adds = [
        "ms_state_bond_interest",
        "ms_other_additions"
    ]

class ms_subtractions_from_agi(Variable):
    """Mississippi subtractions from federal AGI"""
    # Subtract state-specific deductions
    adds = [
        "ms_retirement_income_exclusion",
        "ms_other_subtractions"
    ]

Code Reuse Patterns

Avoid Duplication - Create Intermediate Variables

❌ ANTI-PATTERN: Copy-pasting calculations

# File 1: calculates income after deduction
def formula(household, period, parameters):
    gross = add(household, period, ["income"])
    deduction = p.deduction * household.nb_persons()
    return max_(gross - deduction, 0)

# File 2: DUPLICATES same calculation
def formula(household, period, parameters):
    gross = add(household, period, ["income"])  # Copy-pasted
    deduction = p.deduction * household.nb_persons()  # Copy-pasted
    after_deduction = max_(gross - deduction, 0)  # Copy-pasted
    return after_deduction < p.threshold

✅ CORRECT: Reuse existing variables

# File 2: reuses calculation
def formula(household, period, parameters):
    countable_income = household("program_countable_income", period)
    return countable_income < p.threshold

When to create intermediate variables:

Same calculation in 2+ places
Logic exceeds 5 lines
Reference implementations have similar variable

Federal Aggregator Variables (Summing State Programs)

CRITICAL: Discover Programs by Enumerating State Directories

When building or modifying a federal-level variable that sums state programs (like tanf, which sums all state TANF programs), never search by keyword or naming pattern. State programs use wildly different names that don't match any single pattern.

Example — TANF program names across states:

Standard: al_tanf, ca_tanf, ny_tanf (28 states)
Non-standard: fl_tca, mn_mfip, ia_fip, ct_tfa, md_tca, mi_fip
Completely unique: ar_tea, id_tafi, la_fitap, ma_tafdc, ne_adc, oh_owf, ut_fep, wy_power, ky_ktap, nh_fanf, tn_ff

None of the "completely unique" names contain "tanf". A keyword search for "tanf" misses 11 programs.

Correct discovery method:

List ALL state directories: ls policyengine_us/variables/gov/states/
For EACH of the 51 jurisdictions (50 states + DC), check non-tax subdirectories for the program type
Look for the top-level benefit variable (entity=SPMUnit, has defined_for)
Add it to the aggregator list

Quick command to find all state TANF-like top-level variables:

# Search for SPMUnit variables under state welfare/human services directories
grep -rl "entity = SPMUnit" policyengine_us/variables/gov/states/*/  \
  --include="*.py" | \
  xargs grep -l "defined_for" | \
  # Then manually check which are TANF top-level benefit variables

Cycle Checks When Wiring Up State Programs

Adding a state program to a federal aggregator can create circular dependencies. Common cycles:

Housing cost cycle: State TANF → housing_cost → rent → housing_assistance → hud_annual_income → TANF
- Fix: Use pre_subsidy_rent + other housing components instead of housing_cost
Childcare cycle: State TANF → childcare_expenses → childcare subsidies → SNAP → TANF
- Fix: Use spm_unit_pre_subsidy_childcare_expenses instead of childcare_expenses
Entity broadcast bugs: Person-level and SPMUnit-level arrays mixed in where() — passes unit tests (scalar) but fails microsim (vectorized)
- Fix: Use spm_unit.project(spm_unit_var) to broadcast to person level

After adding programs, always run the microsimulation test — it catches cycles and entity mismatches that unit tests miss.

Register New Programs in `programs.yaml`

When implementing a new state program, add it to the relevant programs.yaml registry as part of the initial implementation. Missing entries block the program from appearing in the metadata API and coverage page.

Applicant vs Recipient Thresholds

When a program has different thresholds for initial applicants vs enrolled recipients (e.g., activity hours, income limits, FPL tiers), model both paths using a boolean enrollment input variable (e.g., is_tanf_enrolled). A single threshold misclassifies one population group.

TANF-Specific Patterns

Study Reference Implementations First

MANDATORY before implementing any TANF:

DC TANF: /variables/gov/states/dc/dhs/tanf/
IL TANF: /variables/gov/states/il/dhs/tanf/
TX TANF: /variables/gov/states/tx/hhs/tanf/

Learn from them:

Variable organization
Naming conventions
Code reuse patterns
When to use adds vs formula

Standard TANF Structure

tanf/
├── eligibility/
│   ├── demographic_eligible.py
│   ├── income_eligible.py
│   └── eligible.py
├── income/
│   ├── earned/
│   ├── unearned/
│   └── countable_income.py
└── [state]_tanf.py

Simplified TANF Rules

For simplified implementations:

DON'T create state-specific versions of:

Demographic eligibility (use federal)
Immigration eligibility (use federal)
Income sources (use federal baseline)

❌ DON'T CREATE:
ca_tanf_demographic_eligible_person.py
ca_tanf_gross_earned_income.py
parameters/.../income/sources/earned.yaml

✅ DO USE:
# Federal demographic eligibility
is_demographic_tanf_eligible
# Federal income aggregation
tanf_gross_earned_income

Avoiding Unnecessary Wrapper Variables (CRITICAL)

Golden Rule: Only create a state variable if you're adding state-specific logic to it!

Understand WHY Variables Exist, Not Just WHAT

When studying reference implementations:

Note which variables they have
READ THE CODE inside each variable
Ask: "Does this variable have state-specific logic?"
If it just returns federal baseline → DON'T copy it

Variable Creation Decision Tree

Before creating ANY state-specific variable, ask:

Does federal baseline already calculate this?
Does my state do it DIFFERENTLY than federal?
Can I write the difference in 1+ lines of state-specific logic?
Will this calculation be used in 2+ other variables? (Code reuse exception)

Decision:

If YES/NO/NO/NO → DON'T create the variable, use federal directly
If YES/YES/YES/NO → CREATE the variable with state logic
If YES/NO/NO/YES → CREATE as intermediate variable for code reuse (see exception below)

EXCEPTION: Code Reuse Justifies Intermediate Variables

Even without state-specific logic, create a variable if the SAME calculation is used in multiple places.

❌ Bad - Duplicating calculation across variables:

# Variable 1 - Income eligibility
class mo_tanf_income_eligible(Variable):
    def formula(spm_unit, period, parameters):
        # Duplicated calculation
        gross = add(spm_unit, period, ["tanf_gross_earned_income", "tanf_gross_unearned_income"])
        return gross <= p.income_limit

# Variable 2 - Countable income
class mo_tanf_countable_income(Variable):
    def formula(spm_unit, period, parameters):
        # SAME calculation repeated!
        gross = add(spm_unit, period, ["tanf_gross_earned_income", "tanf_gross_unearned_income"])
        deductions = spm_unit("mo_tanf_deductions", period)
        return max_(gross - deductions, 0)

# Variable 3 - Need standard
class mo_tanf_need_standard(Variable):
    def formula(spm_unit, period, parameters):
        # SAME calculation AGAIN!
        gross = add(spm_unit, period, ["tanf_gross_earned_income", "tanf_gross_unearned_income"])
        return where(gross < p.threshold, p.high, p.low)

✅ Good - Extract into reusable intermediate variable:

# Intermediate variable - used in multiple places
class mo_tanf_gross_income(Variable):
    adds = ["tanf_gross_earned_income", "tanf_gross_unearned_income"]

# Variable 1 - Reuses intermediate
class mo_tanf_income_eligible(Variable):
    def formula(spm_unit, period, parameters):
        gross = spm_unit("mo_tanf_gross_income", period)  # Reuse
        return gross <= p.income_limit

# Variable 2 - Reuses intermediate
class mo_tanf_countable_income(Variable):
    def formula(spm_unit, period, parameters):
        gross = spm_unit("mo_tanf_gross_income", period)  # Reuse
        deductions = spm_unit("mo_tanf_deductions", period)
        return max_(gross - deductions, 0)

# Variable 3 - Reuses intermediate
class mo_tanf_need_standard(Variable):
    def formula(spm_unit, period, parameters):
        gross = spm_unit("mo_tanf_gross_income", period)  # Reuse
        return where(gross < p.threshold, p.high, p.low)

When to create intermediate variables for reuse:

✅ Same calculation appears in 2+ variables
✅ Represents a meaningful concept (e.g., "gross income", "net resources")
✅ Simplifies maintenance (change once vs many places)
✅ Follows DRY (Don't Repeat Yourself) principle

When NOT to create (still a wrapper):

❌ Only used in ONE place
❌ Just passes through another variable unchanged
❌ Adds indirection without code reuse benefit

Red Flags for Unnecessary Wrapper Variables

❌ INVALID - Pure wrapper, no state logic:
class in_tanf_assistance_unit_size(Variable):
    def formula(spm_unit, period):
        return spm_unit("spm_unit_size", period)  # Just returns federal

❌ INVALID - Aggregation without transformation:
class in_tanf_countable_unearned_income(Variable):
    def formula(tax_unit, period):
        return tax_unit.sum(person("tanf_gross_unearned_income", period))

❌ INVALID - Pass-through with no modification:
class in_tanf_gross_income(Variable):
    def formula(entity, period):
        return entity("tanf_gross_income", period)

Examples of VALID State Variables

✅ VALID - Has state-specific disregard:
class in_tanf_countable_earned_income(Variable):
    def formula(spm_unit, period, parameters):
        p = parameters(period).gov.states.in.tanf.income
        earned = spm_unit("tanf_gross_earned_income", period)
        return earned * (1 - p.earned_income_disregard_rate)  # STATE LOGIC

✅ VALID - Uses state-specific limits:
class in_tanf_income_eligible(Variable):
    def formula(spm_unit, period, parameters):
        p = parameters(period).gov.states.in.tanf
        income = spm_unit("tanf_countable_income", period)
        size = spm_unit("spm_unit_size", period.this_year)
        limit = p.income_limit[min_(size, p.max_household_size)]  # STATE PARAMS
        return income <= limit

✅ VALID - IL has different counting rules:
class il_tanf_assistance_unit_size(Variable):
    adds = [
        "il_tanf_payment_eligible_child",  # STATE-SPECIFIC
        "il_tanf_payment_eligible_parent",  # STATE-SPECIFIC
    ]

Enum Variables with Numeric Ranges Must Have Formulas

When an enum's labels describe numeric ranges (e.g., "30+ hours/week", "20-29 hours/week"), it MUST be a formula variable, not a bare input. The numeric breakpoints should be a bracket parameter, and the formula should derive the category from an existing numeric input variable.

❌ WRONG — bare input enum, user picks manually:

class program_time_category(Variable):
    value_type = Enum
    possible_values = TimeCategory  # "30+ hrs", "20-29 hrs", etc.
    default_value = TimeCategory.FULL_TIME
    # No formula — user must select

✅ CORRECT — derived from a numeric input via bracket parameter:

# time_category.yaml — single_amount bracket maps numeric input to category index
brackets:
  - threshold:
      2024-01-01: 0
    amount:
      2024-01-01: 4  # QUARTER_TIME
  - threshold:
      2024-01-01: 10
    amount:
      2024-01-01: 3  # HALF_TIME
  - threshold:
      2024-01-01: 20
    amount:
      2024-01-01: 2  # THREE_QUARTER_TIME
  - threshold:
      2024-01-01: 30
    amount:
      2024-01-01: 1  # FULL_TIME

class program_time_category(Variable):
    value_type = Enum
    possible_values = TimeCategory
    default_value = TimeCategory.FULL_TIME

    def formula(person, period, parameters):
        hours = person("childcare_hours_per_week", period)
        p = parameters(period).gov.states.xx.agency.program
        return p.time_category.calc(hours)

Before creating any enum variable, check:

Do the labels describe numeric ranges or thresholds?
Does an existing input variable provide the numeric value? (Grep the codebase)
If yes to both → use a bracket parameter + formula, not a bare input

Rate Table Dimensions: One Enum Variable Per Dimension

When a rate table has multiple dimensions (provider type, age group, star rating, authorization level), create a separate Enum variable for each dimension. This enables simple Enum bracket indexing in the rate lookup variable.

Pattern:

# Each dimension is its own Enum variable:
provider_type = person("ri_ccap_provider_type", period)
time_category = person("ri_ccap_time_category", period)
star_rating = person("ri_ccap_star_rating", period)
age_group = person("ri_ccap_center_age_group", period)

# Rate lookup is just Enum indexing:
rate = p.licensed_center[time_category][star_rating][age_group]

When dimensions differ by category, use separate Enums:

Licensed centers have 4 age groups (Infant, Toddler, Preschool, School Age)
Family child care has 3 age groups (Infant/Toddler, Preschool, School Age)
→ Create ri_ccap_center_age_group and ri_ccap_family_age_group

When quality ratings differ by provider type:

Licensed centers/family use star ratings (1-5)
License-exempt uses step ratings (1-4)
→ Create ri_ccap_star_rating and ri_ccap_step_rating as separate Enums

Use Enum (not int) for constrained categories:

# ❌ BAD — star rating as int (what values are valid? unclear)
class ri_ccap_star_rating(Variable):
    value_type = int

# ✅ GOOD — star rating as Enum (self-documenting, validated)
class RICCAPStarRating(Enum):
    STAR_1 = "1 Star"
    STAR_2 = "2 Stars"
    STAR_3 = "3 Stars"
    STAR_4 = "4 Stars"
    STAR_5 = "5 Stars"

If you need helper functions in a rate lookup, your parameter structure is too granular. Restructure the parameter files to use Enum breakdowns (see parameter-patterns skill) so the variable is just select() + indexing.

Reference implementation: MA CCFA reimbursement — ma_ccfa_center_based_early_education_reimbursement.py is 3 lines: get region, get age category, index into parameter.

Enum Ordering and Design

Place NONE/default values at the END of the enum list, not the beginning:

# ❌ BAD:
class CareType(Enum):
    NONE = "None"           # Default first
    NURSING = "Nursing"

# ✅ GOOD:
class CareType(Enum):
    NURSING = "Nursing"
    NONE = "None"           # Default at end

Don't split enum values for different eligibility pathways that share the same payment amount. If the payment table has one row, the enum should have one value — regardless of how many qualification routes exist.

Explicit Category Matching in `select()`

Match ALL categories explicitly — don't rely on default for a specific known category:

# ❌ BAD — default hides bugs if new categories are added:
return select(
    [is_region_a, is_region_b],
    [rate_a, rate_b],
    default=rate_c,  # Region C is implicit
)

# ✅ GOOD — all categories explicit:
return select(
    [is_region_a, is_region_b, is_region_c],
    [rate_a, rate_b, rate_c],
)

State Variables to AVOID Creating

For TANF implementations:

❌ DON'T create these (use federal directly):

state_tanf_assistance_unit_size (unless different counting rules like IL)
state_tanf_countable_unearned_income (unless state has disregards)
state_tanf_gross_income (just use federal baseline)
Any variable that's just return entity("federal_variable", period)

✅ DO create these (when state has unique rules):

state_tanf_countable_earned_income (if unique disregard %)
state_tanf_income_eligible (state income limits)
state_tanf_maximum_benefit (state payment standards)
state_tanf (final benefit calculation)

Demographic Eligibility Pattern

Option 1: Use Federal (Simplified)

class ca_tanf_eligible(Variable):
    def formula(spm_unit, period, parameters):
        # Use federal variable
        has_eligible = spm_unit.any(
            spm_unit.members("is_demographic_tanf_eligible", period)
        )
        return has_eligible & income_eligible

Option 2: State-Specific (Different thresholds)

class ca_tanf_demographic_eligible_person(Variable):
    def formula(person, period, parameters):
        p = parameters(period).gov.states.ca.tanf
        age = person("age", period.this_year)  # NOT monthly_age

        age_limit = where(
            person("is_full_time_student", period),
            p.age_threshold.student,
            p.age_threshold.minor_child
        )
        return age < age_limit

Common Implementation Patterns

Income Eligibility

class program_income_eligible(Variable):
    value_type = bool
    entity = SPMUnit
    definition_period = MONTH

    def formula(spm_unit, period, parameters):
        p = parameters(period).gov.states.xx.program
        income = spm_unit("program_countable_income", period)
        size = spm_unit("spm_unit_size", period.this_year)

        # Get threshold from parameters
        threshold = p.income_limit[min_(size, p.max_household_size)]
        return income <= threshold

Benefit Calculation

class program_benefit(Variable):
    value_type = float
    entity = SPMUnit
    definition_period = MONTH
    unit = USD

    def formula(spm_unit, period, parameters):
        p = parameters(period).gov.states.xx.program
        eligible = spm_unit("program_eligible", period)

        # Calculate benefit amount
        base = p.benefit_schedule.base_amount
        adjustment = p.benefit_schedule.adjustment_rate
        size = spm_unit("spm_unit_size", period.this_year)

        amount = base + (size - 1) * adjustment
        return where(eligible, amount, 0)

Using Scale Parameters

def formula(entity, period, parameters):
    p = parameters(period).gov.states.az.program
    federal_p = parameters(period).gov.hhs.fpg

    # Federal base with state scale
    size = entity("household_size", period.this_year)
    fpg = federal_p.first_person + federal_p.additional * (size - 1)
    state_scale = p.income_limit_scale  # Often exists
    income_limit = fpg * state_scale

Handling Parameter Structure Transitions

When a parameter changes structure over time (e.g., flat rate → marginal brackets), the parameter side uses a boolean toggle with separate files (see parameter patterns skill). The variable must branch on that toggle.

Pattern: Use if p.toggle: to select the right parameter access method:

class wa_capital_gains_tax(Variable):
    value_type = float
    entity = TaxUnit
    definition_period = YEAR
    unit = USD
    defined_for = StateCode.WA

    def formula(tax_unit, period, parameters):
        p = parameters(period).gov.states.wa.tax.income.capital_gains
        taxable_ltcg = ...  # calculation

        # Toggle between flat and bracket-based calculation
        if p.rate.flat_applies:
            return taxable_ltcg * p.rate.flat
        return p.rate.incremental.calc(taxable_ltcg)

Why if (not where) is correct here:

p.rate.flat_applies is a parameter (scalar boolean at a given instant), not a per-entity variable
Python if is appropriate because the entire population uses the same rate structure in a given year
where() is for per-entity branching (e.g., different treatment by filing status)

When to use this pattern:

✅ A flat rate became a marginal bracket schedule at a specific date
✅ A single value became a lookup table at a specific date
✅ Any parameter whose access method (.calc() vs *) changes by period

When NOT to use this pattern:

❌ The parameter structure is the same across all periods (just access it normally)
❌ The branching depends on a per-entity condition like income or age (use where() instead)
❌ A new bracket was added to an existing scale using .inf (no variable changes needed — .calc() works as before; see parameter patterns skill)

Gating Provisions with `in_effect` Boolean

When a provision starts or ends at a specific date, use if p.provision.in_effect: to gate the entire logic block. The parameter side has an in_effect.yaml boolean (see parameter patterns skill). The variable wraps the provision's logic in a plain if block.

Real-world example — CT TFA high-earnings reduction (new in 2024):

class ct_tfa(Variable):
    value_type = float
    entity = SPMUnit
    definition_period = MONTH
    unit = USD
    defined_for = "ct_tfa_eligible"

    def formula(spm_unit, period, parameters):
        p = parameters(period).gov.states.ct.dss.tfa.payment
        payment_standard = spm_unit("ct_tfa_payment_standard", period)
        countable_unearned_income = spm_unit(
            "ct_tfa_countable_unearned_income", period
        )
        raw_benefit = max_(payment_standard - countable_unearned_income, 0)

        # Per CGS § 17b-112(d), high earners have their benefit reduced.
        # This provision did not exist before 2024.
        if p.high_earnings.in_effect:
            gross_earnings = add(
                spm_unit, period, ["tanf_gross_earned_income"]
            )
            fpg = spm_unit("tanf_fpg", period)
            high_income_threshold = p.high_earnings.rate * fpg
            high_income = gross_earnings >= high_income_threshold
            reduction_multiplier = 1 - p.high_earnings.reduction_rate
            return where(
                high_income,
                raw_benefit * reduction_multiplier,
                raw_benefit,
            )
        return raw_benefit

Key points:

if p.high_earnings.in_effect: uses Python if because it's a parameter (scalar boolean), not a per-entity variable
The if block is NEVER entered for periods before the effective date (2024)
Inside the if block, where() IS used for per-entity branching (some households have high income, others don't)
The provision's sub-parameters (rate, reduction_rate) are only accessed inside the guarded block

When to use this pattern:

✅ A new provision is added by legislation at a specific date
✅ An existing provision is repealed at a specific date
✅ The provision gates an entire block of logic with its own sub-parameters

When NOT to use this pattern:

❌ The branching depends on a per-entity condition (use where() instead)
❌ The parameter structure changes (use flat_applies pattern above)

Regional Variation with `regional_in_effect` Boolean

When a program transitions between regional and statewide payment standards, use if p.regional_in_effect: to switch between regional enum lookup and flat parameter access.

Real-world example — CT TFA payment standard (regional before 2022, flat after):

class ct_tfa_payment_standard(Variable):
    value_type = float
    entity = SPMUnit
    definition_period = MONTH
    unit = USD
    defined_for = StateCode.CT

    def formula(spm_unit, period, parameters):
        p = parameters(period).gov.states.ct.dss.tfa.payment
        size = spm_unit("spm_unit_size", period.this_year)
        capped_size = min_(size, p.max_unit_size)

        if p.regional_in_effect:
            region = spm_unit.household("ct_tfa_region", period)
            region_a = region == region.possible_values.REGION_A
            region_c = region == region.possible_values.REGION_C
            return select(
                [region_a, region_c],
                [
                    p.regional.region_a.amount[capped_size],
                    p.regional.region_c.amount[capped_size],
                ],
                default=p.regional.region_b.amount[capped_size],
            )

        return p.amount[capped_size]

Key points:

if p.regional_in_effect: uses Python if — scalar parameter boolean
Inside the if block, select() handles per-entity branching by region enum
The default parameter in select() handles regions not explicitly listed
When regional_in_effect is false, falls through to the flat p.amount[capped_size]
Region enum variable (ct_tfa_region) is only accessed when regional variation is active

When to use this pattern:

✅ Program transitions from regional to statewide standards (or vice versa)
✅ Regional lookup uses an enum variable with select()
✅ The flat and regional parameter trees have different structures

When NOT to use this pattern:

❌ Regional variation always applies (just use regional parameters directly)
❌ The variation is by household characteristic, not geography (use where())

Choosing Between Boolean Toggle Patterns (Summary)

All three patterns use if p.boolean: branching on a scalar parameter. Here's when to use each:

Pattern	Parameter Side	Variable Side	Use Case
`flat_applies`	Folder with flat + bracket + toggle	`if p.flat_applies:` switches access method	Structure changes (flat → brackets)
`in_effect`	Single `in_effect.yaml` + sibling params	`if p.provision.in_effect:` gates logic block	Provision starts/ends at date
`regional_in_effect`	Single boolean + regional/ folder + flat	`if p.regional_in_effect:` switches lookup	Regional ↔ statewide transition

Common rule: All use Python if (not where) because parameters are scalar booleans at a given instant — the entire population uses the same code path for a given period.

See policyengine-parameter-patterns skill for the parameter-side implementation of each pattern.

Parameter-to-Variable Mapping Requirements

Every Parameter Must Have a Variable

CRITICAL: Complete implementation means every parameter is used!

When you create parameters, you MUST create corresponding variables:

Parameter Type	Required Variable(s)
resources/limit	`state_program_resources_eligible`
income/limit	`state_program_income_eligible`
min_work_hours or activity_requirements	`state_program_activity_eligible`
payment_standard	`state_program_maximum_benefit`
income/disregard	`state_program_countable_earned_income`
categorical/requirements	`state_program_categorically_eligible`

Complete Eligibility Formula

The main eligibility variable MUST combine ALL checks:

class state_program_eligible(Variable):
    def formula(spm_unit, period, parameters):
        income_eligible = spm_unit("state_program_income_eligible", period)
        resources_eligible = spm_unit("state_program_resources_eligible", period)  # DON'T FORGET!
        categorical = spm_unit("state_program_categorically_eligible", period)

        return income_eligible & resources_eligible & categorical

Common Implementation Failures:

❌ Created resource limit parameter but no resource_eligible variable
❌ Created min_work_hours parameter but no activity_eligible variable
❌ Main eligible variable only checks income, ignores resources/work/immigration
❌ Parameters created but never referenced in any formula
❌ Spec lists requirements (work hours, citizenship) but no variables implement them

Childcare Subsidy Benefit Calculation

Childcare subsidy programs (CCAP, CCFA, etc.) share a common pattern: the benefit is capped at actual childcare expenses, not just the provider reimbursement rate.

PolicyEngine has an existing federal variable pre_subsidy_childcare_expenses (Person, YEAR) that represents what families actually pay for childcare. Always use this variable — don't calculate the benefit from provider rates alone.

# ❌ BAD — subsidy based only on provider rate (ignores actual expenses)
class ri_ccap(Variable):
    def formula(spm_unit, period, parameters):
        total_weekly_rate = spm_unit.sum(weekly_rate * is_eligible_child)
        return max_(total_weekly_rate - family_share, 0)

# ✅ GOOD — subsidy capped at actual expenses (matches MA CCFA, CO CCAP pattern)
class ri_ccap(Variable):
    def formula(spm_unit, period, parameters):
        actual_expenses = spm_unit(
            "spm_unit_pre_subsidy_childcare_expenses", period.this_year
        )
        copay = spm_unit("ri_ccap_family_share", period)
        max_reimbursement = add(spm_unit, period, ["ri_ccap_weekly_provider_rate"])
        uncapped_benefit = max_(actual_expenses - copay, 0)
        return min_(uncapped_benefit, max_reimbursement)

The subsidy is the lesser of:

Actual expenses minus co-payment
Maximum reimbursement (provider rate based on type, quality, age, time)

name	policyengine-variable-patterns
description	PolicyEngine variable patterns - variable creation, no hard-coding principle, federal/state separation, metadata standards