statsmodels Skill

statsmodels general-purpose statistical modeling library for Python. Covers OLS/WLS/GLS, GLM (logit, probit, Poisson, negative binomial), discrete choice models, time series (ARIMA, SARIMAX, VAR), mixed effects (MixedLM), robust regression, hypothesis tests, and comprehensive diagnostics. Supports R-style formula API. Use when fitting regressions without fixed effects, running GLMs or logit/probit, analyzing time series, or using formula syntax. For fixed effects or DiD, use pyfixest; for panel/IV/system models, use linearmodels.

Comprehensive skill for statistical modeling with statsmodels. Use decision trees below to find the right guidance, then load detailed references.

What is statsmodels?

statsmodels is the general-purpose statistical modeling library for Python:

• Two APIs: Formula API (smf.ols("y ~ x1 + x2", data=df)) for R-style modeling, and array API (sm.OLS(y, X)) for programmatic control
• Broad model coverage: OLS, WLS, GLS, GLM (all families), logit, probit, multinomial, count models, zero-inflated models, quantile regression, robust regression
• Time series: ARIMA, SARIMAX, VAR, exponential smoothing, state space models, unit root tests
• Diagnostics: Heteroskedasticity tests, normality tests, specification tests, VIF, influence measures, residual analysis
• Hypothesis testing: t-tests, F-tests, Wald tests, likelihood ratio tests, multiple comparison corrections

How to Use This Skill

Reference File Structure

File	Purpose	When to Read
quickstart.md	Installation, formula vs array API, first model	Starting with statsmodels
linear-models.md	OLS, WLS, GLS, robust regression, quantile regression	Fitting linear models
glm-discrete.md	GLM families, logit/probit, count models, zero-inflated	Non-linear models, binary/count outcomes
time-series.md	ARIMA, SARIMAX, VAR, exponential smoothing, unit root tests	Analyzing temporal data
diagnostics.md	Heteroskedasticity, normality, VIF, influence, residuals	Checking model assumptions
hypothesis-testing.md	t-tests, F-tests, Wald tests, multiple comparisons	Testing coefficients and comparing models
gotchas.md	Constant term, convergence, predict pitfalls, pyfixest boundary	Debugging issues

Reading Order

1. New to statsmodels? Start with quickstart.md then linear-models.md
2. Need GLM or logit/probit? Read quickstart.md then glm-discrete.md
3. Time series analysis? Read quickstart.md then time-series.md
4. Checking model assumptions? Read diagnostics.md
5. Coming from R? Read quickstart.md (formula API mirrors R syntax)

The reference-file routing in this skill applies to advisory and brainstorming turns as much as implementation. Recommending an approach, reviewing a plan, or answering a question that touches a routed topic calls for reading the routed reference file just as much as writing code does — the reference files carry curated caveats and environment-specific constraints that this overview and general knowledge lack.

Related Skills

• pyfixest: Use instead of statsmodels when your model needs absorbed fixed effects, IV with FE, or difference-in-differences. pyfixest is faster for FE models; statsmodels is broader for everything else
• linearmodels: Use for panel data models (FE, RE, between, first difference, Fama-MacBeth), IV/GMM without FE (2SLS, LIML, GMM), system estimation (SUR, 3SLS), and asset pricing. Built on top of statsmodels; extends it for structured data
• svy: Use for survey-weighted regression and estimation with complex survey designs. Important: statsmodels WLS is NOT equivalent to survey-weighted regression — WLS handles heteroscedastic errors but does not account for stratification, clustering, or finite population corrections. If your data comes from a complex probability survey (NHANES, ACS PUMS, CPS, ECLS-K, etc.), load the svy skill instead
• data-scientist: Provides methodology guidance (when to use which model, assumption checking protocol, interpretation). Load alongside statsmodels for the "why"; statsmodels provides the "how"
• polars: Data manipulation before modeling. statsmodels accepts pandas DataFrames; convert with df.to_pandas() if using Polars
• plotnine: Publication-quality visualization of model results and diagnostics

Quick Decision Trees

"I need to fit a regression model"

What kind of regression?
├─ Linear (continuous outcome)
│   ├─ Basic OLS → ./references/linear-models.md
│   ├─ Weighted least squares → ./references/linear-models.md
│   │   (⚠ WLS ≠ survey-weighted regression — for complex surveys, use `svy` skill)
│   ├─ Correlated errors (GLS) → ./references/linear-models.md
│   ├─ Robust to outliers (M-estimator) → ./references/linear-models.md
│   └─ Quantile regression → ./references/linear-models.md
├─ Binary outcome (0/1)
│   ├─ Logit → ./references/glm-discrete.md
│   └─ Probit → ./references/glm-discrete.md
├─ Count outcome (0, 1, 2, ...)
│   ├─ Poisson → ./references/glm-discrete.md
│   ├─ Negative binomial → ./references/glm-discrete.md
│   └─ Zero-inflated → ./references/glm-discrete.md
├─ Multinomial (3+ categories)
│   └─ Multinomial logit → ./references/glm-discrete.md
├─ GLM (custom family/link)
│   └─ GLM framework → ./references/glm-discrete.md
└─ Need fixed effects?
    └─ Use pyfixest instead (faster FE absorption)

"I need to analyze time series"

What time series task?
├─ Forecast a single series
│   ├─ ARIMA / SARIMAX → ./references/time-series.md
│   └─ Exponential smoothing → ./references/time-series.md
├─ Multiple interrelated series
│   └─ VAR / VECM → ./references/time-series.md
├─ Test for stationarity
│   ├─ ADF test → ./references/time-series.md
│   └─ KPSS test → ./references/time-series.md
├─ Examine autocorrelation
│   └─ ACF / PACF → ./references/time-series.md
└─ Structural time series
    └─ Unobserved components → ./references/time-series.md

"I need to check model assumptions"

What assumption to check?
├─ Heteroskedasticity → ./references/diagnostics.md
│   ├─ Breusch-Pagan test
│   └─ White test
├─ Normality of residuals → ./references/diagnostics.md
│   ├─ Jarque-Bera test
│   └─ Shapiro-Wilk test
├─ Specification / functional form → ./references/diagnostics.md
│   └─ RESET test
├─ Multicollinearity → ./references/diagnostics.md
│   ├─ VIF
│   └─ Condition number
├─ Influential observations → ./references/diagnostics.md
│   ├─ Cook's distance
│   └─ Leverage / DFFITS
├─ Serial correlation → ./references/diagnostics.md
│   └─ Durbin-Watson / Breusch-Godfrey
└─ All of the above → ./references/diagnostics.md

"I need to test hypotheses"

What kind of test?
├─ Single coefficient significance → ./references/hypothesis-testing.md
├─ Joint significance (F-test) → ./references/hypothesis-testing.md
├─ Linear restrictions (Wald) → ./references/hypothesis-testing.md
├─ Compare nested models (LR test) → ./references/hypothesis-testing.md
├─ Multiple comparisons correction → ./references/hypothesis-testing.md
└─ Chi-squared test → ./references/hypothesis-testing.md

"Something isn't working"

Common issues?
├─ Missing constant / intercept → ./references/gotchas.md
├─ Convergence warnings → ./references/gotchas.md
├─ predict() errors → ./references/gotchas.md
├─ Formula parsing issues → ./references/gotchas.md
├─ summary() formatting → ./references/gotchas.md
├─ statsmodels vs pyfixest → ./references/gotchas.md
└─ General troubleshooting → ./references/gotchas.md

File-First Execution in Research Workflows

Important: In data research pipelines (see CLAUDE.md), statsmodels analyses are executed through script files, not interactively. This ensures auditability and reproducibility.

The pattern:

1. Write model code to scripts/stage8_analysis/{step}_{model-name}.py
2. Execute via Bash with automatic output capture wrapper script
3. Validation results get automatically embedded in scripts as comments
4. If failed, create versioned copy for fixes

Closely read agent_reference/SCRIPT_EXECUTION_REFERENCE.md for the mandatory file-first execution protocol covering complete code file writing, output capture, and file versioning rules.

See:

• agent_reference/SCRIPT_EXECUTION_REFERENCE.md — Script execution protocol and format with validation

The examples below show statsmodels syntax. In research workflows, wrap them in scripts following the file-first pattern.

---

Quick Reference

Essential Imports

import statsmodels.api as sm           # Array API
import statsmodels.formula.api as smf  # Formula API (R-style)

Core Operations

Operation	Code
OLS (formula)	smf.ols("y ~ x1 + x2", data=df).fit()
OLS (array)	sm.OLS(y, sm.add_constant(X)).fit()
Logit	smf.logit("y ~ x1 + x2", data=df).fit()
Probit	smf.probit("y ~ x1 + x2", data=df).fit()
Poisson	smf.poisson("y ~ x1 + x2", data=df).fit()
GLM (custom)	smf.glm("y ~ x1", data=df, family=sm.families.Binomial()).fit()
WLS	smf.wls("y ~ x1", data=df, weights=w).fit()
Robust (HC1)	fit = smf.ols(...).fit(cov_type='HC1')
ARIMA	sm.tsa.ARIMA(y, order=(p,d,q)).fit()
Summary	results.summary()
Predict	results.predict(new_data)
Confidence intervals	results.conf_int(alpha=0.05)
Marginal effects	results.get_margeff(at='overall')
VIF	from statsmodels.stats.outliers_influence import variance_inflation_factor
Breusch-Pagan	sm.stats.diagnostic.het_breuschpagan(resid, exog)

Formula Syntax

# Additive terms
"y ~ x1 + x2 + x3"

# Interaction (with main effects)
"y ~ x1 * x2"           # equivalent to x1 + x2 + x1:x2

# Interaction only (no main effects)
"y ~ x1 : x2"

# Categorical variable
"y ~ C(region)"          # treatment coding (default)
"y ~ C(region, Treatment(reference='West'))"  # explicit reference

# Suppress intercept
"y ~ x1 + x2 - 1"

# Polynomial
"y ~ x1 + I(x1**2)"     # I() protects Python operators

Topic Index

Topic	Reference File
Installation	./references/quickstart.md
Formula vs array API	./references/quickstart.md
Reading summary output	./references/quickstart.md
Comparison to pyfixest	./references/quickstart.md
OLS regression	./references/linear-models.md
Weighted least squares	./references/linear-models.md
GLS	./references/linear-models.md
Robust regression (RLM)	./references/linear-models.md
Quantile regression	./references/linear-models.md
Interactions and polynomials	./references/linear-models.md
GLM framework	./references/glm-discrete.md
Logit / probit	./references/glm-discrete.md
Multinomial logit	./references/glm-discrete.md
Poisson / negative binomial	./references/glm-discrete.md
Zero-inflated models	./references/glm-discrete.md
Marginal effects	./references/glm-discrete.md
Exposure / offset	./references/glm-discrete.md
ARIMA / SARIMAX	./references/time-series.md
VAR / VECM	./references/time-series.md
Exponential smoothing	./references/time-series.md
Unit root tests	./references/time-series.md
ACF / PACF	./references/time-series.md
Forecasting	./references/time-series.md
State space models	./references/time-series.md
Heteroskedasticity tests	./references/diagnostics.md
Normality tests	./references/diagnostics.md
Specification tests (RESET)	./references/diagnostics.md
VIF / multicollinearity	./references/diagnostics.md
Influence measures	./references/diagnostics.md
Residual analysis	./references/diagnostics.md
Durbin-Watson	./references/diagnostics.md
t-tests and F-tests	./references/hypothesis-testing.md
Wald tests	./references/hypothesis-testing.md
Likelihood ratio tests	./references/hypothesis-testing.md
Multiple comparison corrections	./references/hypothesis-testing.md
Comparing nested models	./references/hypothesis-testing.md
Serial correlation tests	./references/diagnostics.md
Diagnostic checklist	./references/diagnostics.md
Chi-squared tests	./references/hypothesis-testing.md
Joint significance tests	./references/hypothesis-testing.md
Ordered logit / probit	./references/glm-discrete.md
Mixed effects (MixedLM)	./references/linear-models.md
Constant term pitfall	./references/gotchas.md
Convergence warnings	./references/gotchas.md
predict() issues	./references/gotchas.md
Formula parsing (patsy)	./references/gotchas.md
summary() vs summary2()	./references/gotchas.md
NaN / missing data	./references/gotchas.md
DataFrame index issues	./references/gotchas.md
statsmodels vs pyfixest	./references/gotchas.md

Citation

When this library is used as a primary analytical tool, include in the report's Software & Tools references:

Seabold, S. & Perktold, J. (2010). "Statsmodels: Econometric and Statistical Modeling with Python." Proceedings of the 9th Python in Science Conference.

Cite when: statsmodels is used for GLM estimation, time series modeling, or statistical hypothesis testing central to the analysis. Do not cite when: Only used for post-estimation diagnostics supporting another library's primary estimation.

For method-specific citations (e.g., individual estimators or techniques), consult the reference files in this skill and agent_reference/CITATION_REFERENCE.md.