matlab-analyze-data

name	matlab-analyze-data
description	Analyze data using MATLAB. Use when the task involves tables, timetables, time-series data, numeric arrays, sensor matrices, or gridded data — including but not limited to exploring, filtering, sorting, cleaning, transforming, aggregating, smoothing, padding, trimming, and answering questions about data. MATLAB provides extensive, easy-to-use built-in functions for these workflows with no additional products required.
license	https://www.mathworks.com/content/dam/mathworks/license/pmrl/license.md
metadata	{"author":"MathWorks","version":"1.3"}

MATLAB Data Analysis

Generate idiomatic MATLAB code for tabular data analysis tasks using tables and timetables.

When to Use

• Any task involving tabular data: exploring, cleaning, transforming, or aggregating tables
• Time-series analysis: resampling, synchronizing, trend detection, smoothing
• Answering questions about data in tables (top-N, filtering, group comparisons)
• Data cleaning: missing values, outliers, type conversion, normalization

When NOT to Use

• The task has no tabular data context (no tables, timetables, or structured datasets)
• The primary goal is visualization or plotting, not data analysis
• The task is purely symbolic math, simulation, or app building

This skill covers core MATLAB functions for tabular and time-series workflows. These functions work natively with table and timetable, handle missing data correctly, and are performance-optimized. Prefer the modern functions recommended here (e.g., groupsummary, datetime, fillmissing) over legacy alternatives (e.g., accumarray, nanmean, datenum). Override only if the user explicitly requests otherwise.

Before writing code, read the reference file linked at the end of the relevant section below. Reference files contain correct syntax, common pitfalls, and "Avoid" patterns that prevent silent bugs. Skipping the reference risks using a deprecated approach or hitting a known pitfall.

Key Functions — Available From

Most functions in this skill are available in R2023a or earlier. The following require a newer release:

Function	Available From	Purpose
paddata, trimdata, resize	R2023b	Pad, trim, or resize arrays to target length
clip	R2024a	Clamp values to a range
summary (enhanced)	R2024b	Supports arrays (numeric, datetime, duration, logical); adds Statistics, DataVariables, Detail name-value args
isapprox	R2024b	Tolerance-aware floating-point comparison (use instead of == for computed values)
isbetween (numeric)	R2024b	Check elements within a numeric range
numunique	R2025a	Count distinct values in a variable
allbetween	R2025a	Validate all values are within a range
allunique	R2025a	Validate all values are unique

Getting Oriented with Data

When data is already in a workspace variable, start by understanding its structure and contents. Use JSON output for reliable parsing — table display is designed for human-readable grids, but as text it is easy to misinterpret which values belong to which variables:

jsonencode(summary(T))      % per-variable stats as nested struct
jsonencode(head(T))         % first 8 rows as structured JSON

When getting oriented with unknown data, summary(T) already contains per-variable type, size, NumMissing, and — for numeric/datetime/duration — Min, Max, Mean, Median, Std. For categoricals it includes category names and counts. This is usually sufficient for an initial overview.

If producing a standalone script, leave the semicolon off to invoke the display method — it shows dimensions, variable names, and a truncated preview. Avoid disp (omits headers and prints every row, flooding output on large tables) and fprintf in a loop (verbose, old-style):

summary(T)                  % types, ranges, missing counts per variable
size(T)                     % [nRows, nVars]
sum(ismissing(T))           % missing count per variable
T                           % dimensions + header + truncated preview

For Pearson correlation, use corrcoef (base MATLAB) with Rows="complete" to handle NaN: corrcoef(T{:,vartype("numeric")}, Rows="complete"). For Kendall or Spearman rank correlation, use corr (requires Statistics Toolbox): corr(X, Type="Spearman").

Systematic exploration checklist (distributions, cardinality, duplicates, outlier screening, groupcounts, corrcoef, time-based checks): exploration.md

Data Types

Use modern MATLAB types. These are faster, more readable, and work better with table functions.

Instead of	Use	Why
datenum, datestr	datetime	Proper arithmetic, timezone support
char, cellstr, strcmp	string, ==/matches	== for scalar, matches for vector comparison
Numeric codes or strings with few unique values	categorical	Self-documenting, works with grouping functions, memory-efficient

dt = datetime("2024-01-15",TimeZone="America/New_York");
names = ["Alice" "Bob"];           % not {'Alice', 'Bob'}
T.Status = categorical(T.Status);  % not numeric codes

Use ordinal categorical for ordered data like rankings or severity levels:

T.Priority = categorical(T.Priority, ...
    ["Low" "Medium" "High" "Critical"], Ordinal=true);
urgent = T(T.Priority >= "High",:);

Extract datetime components for computed variables, filtering, or display:

T.Month = month(T.Date);                   % numeric (1-12)
T.Weekday = weekday(T.Date);
T.MonthStart = dateshift(T.Date,"start","month");

String arrays support search, edit, and extraction:

T.Domain = extractAfter(T.Email,"@");
T.Status = replace(T.Status,"N/A","Unknown");
T.Name = strip(T.Name);

Manage categorical levels with mergecats, renamecats, removecats, reordercats:

T.Region = mergecats(T.Region,["Northeast" "Southeast"],"East");
T.Size = reordercats(T.Size,["Small" "Medium" "Large"]);

Full examples and "avoid" patterns: data-types.md

Tables and Timetables

Tables are the primary container for tabular data. Each table variable can be single-column or multi-column (e.g., a matrix); the only requirement is consistent row count. Use "variable" (not "column") to match MathWorks documentation. Prefer dot notation and named access over numeric indexing.

Table orientation: each variable (column) holds values of the same type, units, and meaning; each row is one observation. If data arrives transposed (measurements as rows, subjects as columns), restructure — don't store heterogeneous quantities (e.g., Height and Weight) in one variable, because grouping, filtering, and math operations all assume variables are homogeneous.

It is rarely better to use a for loop to iterate over table variables. MATLAB's table functions operate on multiple variables at once via DataVariables and vartype — prefer these over column-by-column loops.

val = T.Value;                          % dot notation for a single variable
subset = T(:,["A" "B" "C"]);           % parentheses for table subsets
matrix = table2array(T(:,vartype("numeric")));   % extract as array
numericVars = T(:,vartype("numeric"));  % vartype for type-based selection

Fix variable types after import with convertvars:

T = convertvars(T,["Region" "Status"],"categorical");
T = convertvars(T,vartype("cellstr"),"string");

Use timetable when your data has timestamps. If a table has a datetime variable representing when each row was observed, convert it to a timetable. This unlocks time-aware operations that would otherwise require manual date logic:

TT = table2timetable(T,RowTimes="Timestamp");
% Now you can:
daily = retime(TT(:,vartype("numeric")),"daily","mean"); % resample (numeric vars only)
TT = retime(TT, unique(TT.Time), "firstvalue");  % resolve duplicate timestamps
TT2 = synchronize(TT_a,TT_b,"hourly");          % align two time series
TT_range = timerange("2024-01-01","2024-06-01");
subset = TT(TT_range,:);                        % filter by date range
TT_prev = lag(TT,1);                            % time-shift data

Another benefit of timetables: functions like fillmissing, smoothdata, and isoutlier automatically use row times for spacing-aware computation. With a plain table, you need to pass SamplePoints="TimeVar" explicitly to get the same behavior.

If working with legacy timeseries objects, consider converting with timeseries2timetable(ts) — the modern timetable is recommended.

When you want to tag or annotate timetable rows with events, episodes, or phases (sensor anomalies, storms, maintenance windows, warning periods), use eventtable to attach event information to the timetable — do NOT add boolean columns, string labels, or categorical state variables to the timetable itself. The eventtable system keeps event metadata separate from measured data, enables event-based filtering (eventfilter), tolerance matching (withtol), automatic event overlays in stackedplot (no manual subplot+patch/xline plumbing), and timetable display annotation, and avoids polluting the timetable with sparse columns that are mostly missing. To push events from an attached eventtable to the main timetable, use syncevents.

retime, synchronize, lag, timerange, withtol, SamplePoints, ReplaceValues details: tables-and-timetables.md eventtable, eventfilter, syncevents, extractevents details: eventtables.md

Data Cleaning

Missing values

Never compare with == for missing values (NaN == NaN is false). Use ismissing or isnan. For a quick boolean check, use anymissing — more readable and performant than any(ismissing(...)):

anymissing(T.Value)                    % true/false: any missing values?
sum(ismissing(T))                      % missing count per variable (numeric vector)
summary(T, Statistics="nummissing")    % missing counts with variable labels (R2024b+)

Standardize first, then fill. Real data often uses sentinel values ("N/A", "", -999, 0 where zero is meaningless) that MATLAB doesn't recognize as missing:

T = standardizeMissing(T,{"N/A", "null", "", -999});   % convert to standard missing
sum(ismissing(T))                                        % now these show up

Choose a fill method that matches your data. Operate on the whole table with DataVariables to target specific columns rather than extracting individual columns:

T = fillmissing(T,"constant","Unknown", DataVariables="Status");     % categorical default
T = fillmissing(T,"median", DataVariables=vartype("numeric"));       % column median
T = fillmissing(T,"linear", DataVariables="Temperature");            % smooth numeric
T = fillmissing(T,"previous", DataVariables="Setting");              % stepwise data
T = fillmissing(T,"movmedian",hours(2), ...                          % noisy, time-based
    DataVariables="Sensor", SamplePoints="Time");

For mixed-type tables, use vartype to apply different methods by type:

T = fillmissing(T,"linear", DataVariables=vartype("numeric"));
T = fillmissing(T,"previous", DataVariables=vartype("categorical"));

Use MaxGap to avoid interpolating over long stretches of missing data. MaxGap is measured in sample-point units — for timetables, use a duration or calendarDuration:

TT = fillmissing(TT,"linear", MaxGap=hours(24), DataVariables="Loss");

Be cautious with rmmissing on an entire table — it drops any row that has a missing value in any column, which can discard valid data unnecessarily. Prefer handling missingness per-variable with fillmissing or targeted column selection. Use rmmissing when you genuinely need complete cases across all columns.

Outliers and range checking

Consider the data's domain expectations when choosing a detection method. The default ("median") flags values more than 3 scaled MAD from the median:

isOut = isoutlier(T,"quartiles", DataVariables="Value");     % IQR method
isOut = isoutlier(T,"mean", ThresholdFactor=2, DataVariables="Value"); % 2 std from mean
Tclean = rmoutliers(T, DataVariables="Value");               % remove outlier rows (default: median)
T = filloutliers(T,"linear","movmedian",5, DataVariables="Value"); % interpolate over local outliers

Detection methods: "median" (default), "mean", "quartiles", "percentiles", "grubbs", "gesd", "movmedian", "movmean". Use ThresholdFactor to control sensitivity — it sets the number of scaled MADs ("median"), standard deviations ("mean"), or IQR multiplier ("quartiles"). Default is 3 for median/mean, 1.5 for quartiles.

Use OutputFormat="tabular" when detecting across multiple variables. Detection functions (isoutlier, islocalmax, islocalmin, ischange, ismissing) return a plain logical matrix by default on tables — variable names are lost. Pass OutputFormat="tabular" to get a table of logicals you can index by name: isOut = isoutlier(T, OutputFormat="tabular", DataVariables=vars); T(isOut.Revenue, :).

Range operations: check, validate, or clamp values to a range:

tf = isbetween(T.Age,18,65);                    % which rows are in range (R2024b+ for numeric)
allbetween(T.Age,0,120)                          % validate: all values plausible? (R2025a+)
T = clip(T,0,100, DataVariables="Score");        % clamp Score to [0, 100] (R2024a+)

Aggregation statistics and missing values

Most aggregation functions (mean, sum, std, min, max, median) accept "omitmissing" to skip missing values. Prefer "omitmissing" over "omitnan" — it handles numeric data identically but also works with datetime, duration, string, and categorical types. Avoid legacy nanmean/nanstd (which require Statistics Toolbox).

m = mean(T.Value,"omitmissing");

Pitfall with min/max: these take an optional second argument for comparison, so max(x,"omitmissing") tries to compare x with the string. Use the three-argument form:

mx = max(x,[],"omitmissing");         % correct
mn = min(x,[],"omitmissing");         % correct
% max(x,"omitmissing")               % WRONG - errors

Pitfall with std/var: the first optional argument is a weight flag (0=sample, 1=population), not a dimension. To specify dimension, pass the weight first: std(x,0,2). Writing std(x,2) does not compute std along dimension 2.

fillmissing methods, filloutliers options, isoutlier detection methods: data-cleaning.md

Data Transformation

Row filtering and sorting

Prefer isbetween over manual >= & <= for range checks. It handles boundary semantics (open/closed intervals), works consistently across numeric, datetime, and duration types, and is less error-prone than compound expressions:

Thigh = T(T.Value > 100,:);                       % logical indexing (single bound)
TBob = T(T.Name == "Bob",:);                      % equality
Trange = T(isbetween(T.Age,18,65),:);             % range filtering (two bounds)
T = sortrows(T,"Date");                           % ascending by Date
T = sortrows(T,["Group" "Value"],["ascend" "descend"]);  % multi-key sort
top5 = topkrows(T,5,"Sales");                     % top 5 by Sales (descending)

Binning

edges = [0 18 35 50 Inf];
labels = ["Child" "Young Adult" "Adult" "Senior"];
T.AgeGroup = discretize(T.Age,edges,categorical(labels));

Note: if binning is for a subsequent groupsummary, groupfilter, grouptransform, or pivot, those functions support binning on the fly - no need to create a binned column first. See Grouping and Aggregation.

Normalization and scaling

xnorm = normalize(x);                                       % z-score (default)
xnorm = normalize(x,"range");                               % scale to [0, 1]
xnorm = normalize(x,"norm",Inf);                            % divide by max (scales to [0,1] for positive data)
T = normalize(T,DataVariables=vartype("numeric"));           % all numeric variables
T = normalize(T,"zscore", DataVariables="Value");            % specific variable

Transform and manage variables

Check current types with T.Properties.VariableTypes (also writeable as a shortcut for conversion).

T = convertvars(T,"Status","categorical");               % string to categorical
T = convertvars(T,vartype("cellstr"),"string");           % cellstr to string
T = convertvars(T,@(x) isstring(x)||iscellstr(x),@lower); % 2nd arg (vars) and 3rd arg (dataType) can be function handles: select + transform in place
T.Score = round(T.Score);                                 % for a single known variable, use direct dot-assignment, not convertvars
T = renamevars(T,"OldName","NewName");
T = movevars(T,"Key", Before="Value");
T = addvars(T,x,y, Before="Value", NewVariableNames=["X" "Y"]);
T = removevars(T,["Temp1" "Temp2"]);
T = splitvars(T,"Coords", NewVariableNames=["X" "Y"]);    % split multicolumn variable
T = mergevars(T,["X" "Y"], NewVariableName="Coords");     % merge into multicolumn

Adding computed variables

Prefer vectorized table arithmetic where possible. For iterative code where each row depends on the previous, extract variables into arrays, compute in a helper function, and assign back — do not index T.Var(i) inside a loop.

Pass tables directly to math functions (std, mean, sum, log10, etc.) — do not extract with T{:,:} or table2array for operations that accept tables natively. Use std(T(:,vars)) not std(T{:,:}). Only extract to array for functions that require it (e.g., eig, svd, corrcoef).

T.Total = T.A + T.B + T.C;                               % vectorized arithmetic
T.BMI = T.Weight ./ (T.Height / 100).^2;                  % element-wise ops
Tsum = sum(T(:,["A" "B" "C"]),2);                         % math functions work on tables: sum, mean, max, etc.
colStd = std(T(:,["A" "B" "C"]));                         % column-wise std — returns a table
T.Result = rowfun(@myFcn, T, ...                           % complicated row operations
    InputVariables=["A" "B" "C"], OutputFormat="uniform");

Reshaping and aggregation

Choose based on whether you need aggregation, reshaping, or both:

• groupsummary - aggregate only (no reshape): multiple methods, multiple data variables. See Grouping and Aggregation.
• unstack - reshape only (tall to wide, inverse of stack): spread one variable into many
• pivot - aggregate AND reshape (one row per X, one variable per Y): one data variable, one method, multiple grouping variables

% Reshape without aggregation — use unstack
Twide = unstack(Ttall,"Value","Category");

% Aggregate and reshape — use pivot
P = pivot(T, Rows="Category", Columns="Region", DataVariable="Sales", Method="sum");

• stack - gather multiple variables into one (wide to tall):

  Ttall = stack(T,["Q1" "Q2" "Q3" "Q4"], NewDataVariableName="Sales", IndexVariableName="Quarter");
  

• rows2vars - transpose a table (rows become variables)

Joining

T = innerjoin(T1,T2, Keys="Key");
T = outerjoin(T1,T2, Keys="Key", MergeKeys=true);

topkrows, varfun, splitvars/mergevars, reshape examples: data-transformation.md

Grouping and Aggregation

groupsummary is the go-to for grouped statistics. Do not use findgroups+accumarray or manual loops for aggregation — groupsummary is faster and works directly with tables. Use findgroups alone only when you need group indices without aggregation.

G = groupsummary(T,"Category",["mean" "std"],"Value");        % multiple methods on one variable
G = groupsummary(T,["Category" "Region"],"mean","Value");     % multiple grouping vars

Notes:

• Output always includes GroupCount - no need to specify a count method separately. For counts only, use groupcounts.
• Valid method names: "mean", "sum", "std", "min", "max", "median", "mode", "var", "range", "nummissing", "numunique", "nnz", "all". Do not use "numel" or "counts" (these will error).
• Prefer string method names over function handles (e.g., "mean" not @mean). Named methods use accelerated code paths and are significantly faster on large datasets.
• Consider IncludeMissingGroups=false to exclude groups defined by a missing value (such as NaN for numeric types) that can dominate results.
• Use IncludeEmptyGroups=true to include all categories of a categorical variable, even those with no rows.
• Supports on-the-fly binning: groupsummary(T,"Age",[0 18 35 50 Inf],"mean","Income") - no need for discretize first. Works with groupcounts, groupfilter, and grouptransform too.
• Time binning has two forms — choose carefully. Sequential ("hour", "month", "year") creates one bin per calendar period in the data (e.g., Jan 2023, Feb 2023, ...). Cyclic ("hourofday", "dayofweek", "monthofyear") collapses across the higher unit to reveal repeating patterns (e.g., all Mondays together). For example, use "monthofyear" (cyclic) for seasonal patterns; use "month" (sequential) for a timeline. No need to extract components with hour()/month() first — pass the binning rule directly to groupsummary.
• Multiple binning methods: use a cell array when types are mixed (e.g., a named method and custom edges), or a string array when all are named methods:

  G = groupsummary(TT,["Time" "Time"],["year" "month"],"mean","Value");   % all named — string array
  G = groupsummary(T,["Region" "Age"],{"none" [0 18 35 50 Inf]},"mean","Income");  % mixed — cell array
  

  Variable name inputs (grouping variables, data variables) must be string arrays — not cell arrays. See for the general rule.

groupfilter filters rows based on group properties. Two use cases:

% (a) Keep entire groups meeting a condition (e.g., groups with enough data)
T = groupfilter(T,"Category",@(x) numel(x) >= 10);

% (b) Filter individual rows within each group (e.g., per-group outlier removal)
T = groupfilter(T,"Category",@(x) ~isoutlier(x),"Value");

When the filter logic matches a built-in detection function (isoutlier, ismissing, ischange), use it inside the function handle rather than reimplementing the arithmetic. Built-in functions handle edge cases (NaN, constant groups) and accept tuning parameters like ThresholdFactor.

grouptransform transforms data within each group, returning a same-size result (normalize, fill, center, or custom). Use ReplaceValues=false to keep the original column and append the result as a new variable — do not overwrite the original when both raw and transformed values are needed:

T = grouptransform(T,"Category","zscore","Value");                        % overwrites Value
T = grouptransform(T,"Category","zscore","Value",ReplaceValues=false);    % appends zscore_Value

pivot for cross-tabulation:

P = pivot(T, Rows="Category", Columns="Region");                                % counts
P = pivot(T, Rows="Category", Columns="Region", DataVariable="Sales", Method="sum");  % aggregation

Binning rules, groupfilter/grouptransform use cases, pivot options: grouping-and-aggregation.md

Smoothing, Trends, and Patterns

smoothdata is the unified entry point for smoothing (not smooth, which requires Curve Fitting Toolbox):

ysmooth = smoothdata(y,"movmean",5);
ysmooth = smoothdata(y,"gaussian",10);
ysmooth = smoothdata(y,"sgolay",11, Degree=3);       % Savitzky-Golay
ysmooth = smoothdata(y,"movmedian",7);               % robust to outliers

% Target specific variables in a table/timetable
T = smoothdata(T,"movmean",5, DataVariables="Value");

Window size formats: The window can be a scalar or a 2-element vector:

• Scalar k: total window length (e.g., smoothdata(y,"movmean",5) uses 5 elements total)
• 2-element vector [kb kf]: elements before and after the current point (e.g., smoothdata(y,"movmean",[2 2]) uses 2 before + current + 2 after = 5 elements)

Pitfall with time-stamped data: When smoothing a timetable or using SamplePoints with datetime or duration values, the window must be a duration, not a number. Sort by time first — SamplePoints must be ascending:

TT = sortrows(TT);
TT = smoothdata(TT,"movmean",days(30), DataVariables="Value");
% WRONG: smoothdata(TT,"movmean",5, ...)  — numeric window errors with time data

Trends:

ydetrend = detrend(y);                                % remove linear trend
ydetrend = detrend(y,2);                              % remove quadratic trend
[LT,ST,R] = trenddecomp(TT.Value);                   % separate trend + seasonality

Pattern detection:

isPeak = islocalmax(y,MinProminence=5);               % local peaks
isValley = islocalmin(y);                             % local valleys
changes = ischange(y,"mean");                         % mean shift points
changes = ischange(y,"linear");                       % slope/trend direction changes
changes = ischange(y,"variance");                     % variance change points

smoothdata methods, trenddecomp options, ischange details: smoothing-and-trends.md

Array and Grid Data

Use arrays when data is homogeneous numeric AND either (a) naturally 2D/grid (sensors, geospatial), (b) performance-critical inner loop, or (c) upstream tooling delivers arrays. Otherwise, convert to table for metadata and named access.

Dimension pitfalls

std and var take a weight as the SECOND argument — not a dimension. movstd and movvar take a weight as the THIRD argument — not a dimension. Always pass weight explicitly before dimension:

std(X,0,2)         % weight=0, dim=2 (per-row std). NOT std(X,2)
var(X,0,2)         % weight=0, dim=2 (per-row var). NOT var(X,2)
movstd(X,k,0,2)   % window=k, weight=0, dim=2. NOT movstd(X,k,2)
movvar(X,k,0,2)   % window=k, weight=0, dim=2. NOT movvar(X,k,2)

Array stats functions do NOT skip NaN automatically. Pass "omitnan" explicitly: mean(X,1,"omitnan"), std(X,0,1,"omitnan").

2D operations for grids

For 2D grid data (sensor matrices, geospatial fields), use the dedicated 2D functions — do not loop 1D functions over rows/columns:

Task	1D (per column)	2D (grid)
Smooth	smoothdata	smoothdata2
Fill missing	fillmissing	fillmissing2
Find peaks	islocalmax	islocalmax2
Find valleys	islocalmin	islocalmin2

S = smoothdata2(X,"gaussian",{5,5});              % 2D Gaussian smoothing
F = fillmissing2(X,"natural");                    % 2D natural neighbor interpolation
TF = islocalmax2(X,MinProminence=10);             % 2D peak detection

Array-native utilities

Use paddata/trimdata/resize to align arrays — not manual indexing or NaN concatenation:

B = paddata(X,100);                 % pad to 100 rows
B = trimdata(X,50);                 % trim to 50 rows
B = resize(X,100);                  % pad or trim as needed

Use mink/maxk for k smallest/largest — not sort followed by indexing:

[vals,idx] = mink(X,5);            % 5 smallest per column with indices
[vals,idx] = maxk(X,5);            % 5 largest per column with indices

Use bounds for simultaneous min and max:

[lo,hi] = bounds(X);               % min and max per column in one call

Dimension-aware operations, 2D functions, grouping on arrays, resizing: array-and-grid-data.md

Answering Questions About Data

Strategies for producing correct answers when querying tabular data:

• Top/Bottom N: Use topkrows(T,5,"Sales") — handles missing values automatically (NaN/NaT placed last). Use sortrows with MissingPlacement only for multi-step workflows where you need the full sorted table afterward. Think about sort direction — "highest rank" means rank #1 (lowest number), "highest salary" means largest number.
• Cross-variable lookups ("ages of the top 4 by pregnancies"): sort by the ranking variable, read the answer variable from the first N rows.
• Missing data: Watch for sentinel values (0, -999, "N/A") — use standardizeMissing. Set IncludeMissingGroups=false when NaN groups dominate. Never apply rmmissing to an entire table just to answer a question about one variable.
• Return data as stored. Don't substitute or map values unless asked.
• Filtering: Consider exact (==, matches) vs partial (contains, startsWith) matching. Count with height(filtered) or nnz(logicalIdx).

Full strategies and examples: answering-data-questions.md

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