backtesting-configuration

name	backtesting-configuration
description	Configures TimeSeriesFold parameters for backtesting based on deployment scenarios. Maps business requirements (retraining frequency, forecast horizon, data budget) to cross-validation strategy parameters. Use when the user describes how they plan to deploy or evaluate a model.

Backtesting Configuration

When to Use

Use this skill to translate a deployment scenario (retraining cadence, forecast horizon, data budget, ingestion delay) into TimeSeriesFold parameters. TimeSeriesFold is the cross-validation strategy passed via the cv argument to backtesting_forecaster (and its multi-series / stats variants) and to the hyperparameter search functions.

from skforecast.model_selection import backtesting_forecaster, TimeSeriesFold

cv = TimeSeriesFold(
    steps=7,                       # forecast horizon
    initial_train_size=365,        # first training window (required)
    refit=False,                   # train once (default)
    fixed_train_size=True,         # rolling window (default)
    gap=0,
)

metric, predictions = backtesting_forecaster(
    forecaster=forecaster,
    y=data['target'],
    cv=cv,
    metric='mean_absolute_error',
)

For fast hyperparameter tuning where multi-step realism is not required, use OneStepAheadFold instead: it validates one step ahead (no recursive prediction), so it is much faster but less representative of multi-step performance. Use TimeSeriesFold for realistic multi-step backtesting.

Related skills

• Prerequisite: forecasting-single-series / forecasting-multiple-series (have a fitted forecaster before backtesting)
• Alongside: metric-selection (choose the metric(s) backtesting reports)
• Alongside: hyperparameter-optimization (the same cv object drives the search functions)
• Next: prediction-intervals (add interval= / interval_method= to backtest uncertainty)

Stop Conditions

Scan before writing code. Each row lists a rule, the symptom when it is broken, and the recovery. Full pitfall catalog: the troubleshooting-common-errors skill.

Rule	Symptom	Recovery
initial_train_size must be provided (the API default is None, which only works when reusing an already-fitted forecaster)	ValueError / no training in the first fold	Pass an int, date string, or Timestamp, e.g. initial_train_size=len(y) - 100
initial_train_size does not accept a float fraction	ValueError: must be int, date string, Timestamp, or None	Convert fractions to an int, e.g. int(len(data) * 0.7)
Configuration must yield at least 2 folds	Single-fold or empty backtest	Ensure initial_train_size + gap + 2 * steps <= n_observations
refit=True retrains every fold (slow path); the default is refit=False	Backtest much slower than expected	Use refit=False or an int cadence (e.g. refit=7) unless per-fold retraining is required
gap, steps, and fold_stride count observations, not calendar time	Off-by-frequency delays	Convert the delay to the series frequency (e.g. 2 days at daily freq = gap=2)

TimeSeriesFold Parameters

Parameter	Type	Default	Description
initial_train_size	int, str, pd.Timestamp	None (required)	Observations for initial training. Int = count, str/Timestamp = last training date. A common starting point is int(len(data) * 0.7).
refit	bool, int	False	Refit every fold (True), train once (False, default), or every n folds (int).
fixed_train_size	bool	True	Rolling fixed window (True, default) vs expanding window (False).
gap	int	0	Observations between training end and test start.
fold_stride	int, None	None (= steps)	Distance between consecutive test set starts.
skip_folds	int, list, None	None	Skip folds to reduce compute. Int = keep every n-th.
allow_incomplete_fold	bool	True	Allow final fold with fewer observations than steps.

Constraints

• Must produce at least 2 folds: initial_train_size + gap + 2 * steps <= n_observations
• initial_train_size must be large enough for the model to learn patterns (at minimum 2× the lag order for ML models, or 2× steps for statistical models)
• gap simulates real-world delay between data availability and forecast usage
• When fixed_train_size=True, the window rolls forward (oldest data discarded). Use for concept drift or when old data is less relevant.
• When fixed_train_size=False, the window expands (all history retained). Use when more data always helps.

Business Scenario Mapping

Retraining frequency

Scenario	Configuration
Retrain every time new data arrives	refit=True
Retrain weekly (with daily forecasts, steps=1)	refit=7
Retrain monthly (with daily forecasts, steps=7)	refit=4 (every 4 folds × 7 steps ≈ monthly)
Never retrain / train once, evaluate across time	refit=False (fixed_train_size has no effect without refit)

Data freshness vs volume

Scenario	Configuration
Recent data more relevant (concept drift)	fixed_train_size=True
All historical data valuable	fixed_train_size=False (expanding)
Limited compute budget	refit=False or skip_folds=3 (keep every 3rd fold)

Deployment gap

Scenario	Configuration
Real-time predictions (no delay)	gap=0
1-day delay between data collection and forecast	gap=1 (if freq=daily)
Forecast must be ready before weekend (2-day gap)	gap=2

Initial training size

Scenario	Configuration
Default (balanced)	int(len(data) * 0.7)
Maximize evaluation coverage	Minimum viable: 2 * max_lag or 2 * steps
Maximize training data	n_observations - gap - 2 * steps (minimum 2 folds)
Start from specific date	initial_train_size="2023-01-01"
Conservative (large training set)	int(len(data) * 0.8)

Fold stride (test set overlap)

Scenario	Configuration
Non-overlapping evaluation (default)	fold_stride=None (equals steps)
Sliding window evaluation (1-step shift)	fold_stride=1
Sparse evaluation (save compute)	fold_stride=steps * 2

Examples

"I retrain my model every Monday and forecast the next 7 days"

refit = True
fixed_train_size = False  # Keep all history
gap = 0  # No delay
fold_stride = None  # Non-overlapping weeks

"I want to simulate deploying once and seeing how the model degrades"

refit = False  # Train once; fixed_train_size has no effect without refit
gap = 0

"There's a 2-day lag between data ingestion and when forecasts are needed"

gap = 2
refit = True

"I have limited compute — evaluate every other week"

refit = True
skip_folds = 2  # Keep every 2nd fold

"I want maximum evaluation coverage with a 12-step horizon"

initial_train_size = <minimum viable>  # 2 * max_lag
refit = False  # Faster; trains once, fixed_train_size has no effect
fold_stride = 1  # Sliding window (many overlapping folds)