factor_interaction

factor_interaction(data, var2=None, *, ref=None, ref2=None, bin=None, bin2=None)

Fixest-style i() operator for categorical encoding with interactions.

Parameters

Name	Type	Description	Default
data	array - like	The categorical variable to encode.	required
var2	array - like	Optional second variable to interact with (continuous or categorical).	`None`
ref	Hashable	Reference level to drop from `data`.	`None`
ref2	Hashable	Reference level to drop from `var2` (only if `var2` is categorical).	`None`
bin	dict	Mapping of `new_level -> [old_levels]` for binning `data`.	`None`
bin2	dict	Mapping of `new_level -> [old_levels]` for binning `var2`.	`None`

Returns

Name	Type	Description
	FactorValues	The encoded factor values, ready for use in a formulaic model matrix.

Examples

Implements the i() operator and is used by writing i(...) in a formula rather than by calling it directly. Expands a categorical variable into indicators, optionally dropping a reference level and interacting it with a second variable. Commonly used for event study specifications. See the formula syntax tutorial.

import pyfixest as pf

data = pf.get_data()

fit = pf.feols("Y ~ i(f1, ref=0)", data)
fit.tidy().head()

	Estimate	Std. Error	t value	Pr(>\|t\|)	2.5%	97.5%
Coefficient
Intercept	-0.486859	0.319489	-1.523869	1.278682e-01	-1.113830	0.140112
f1::1.0	2.544288	0.430799	5.905979	4.848718e-09	1.698881	3.389695
f1::2.0	-1.634681	0.451825	-3.617948	3.123025e-04	-2.521351	-0.748011
f1::3.0	-0.463981	0.465731	-0.996241	3.193818e-01	-1.377939	0.449978
f1::4.0	-1.762712	0.455041	-3.873741	1.143851e-04	-2.655693	-0.869731

Interacting with a continuous variable gives group-specific slopes.

pf.feols("Y ~ i(f1, X2, ref=0)", data).tidy().head()

	Estimate	Std. Error	t value	Pr(>\|t\|)	2.5%	97.5%
Coefficient
Intercept	-0.163916	0.071887	-2.280195	0.022813	-0.304987	-0.022844
f1::1.0:X2	-0.118795	0.096590	-1.229891	0.219037	-0.308346	0.070755
f1::2.0:X2	-0.193556	0.106548	-1.816610	0.069586	-0.402647	0.015535
f1::3.0:X2	-0.173543	0.114664	-1.513488	0.130482	-0.398562	0.051476
f1::4.0:X2	-0.055993	0.108040	-0.518260	0.604395	-0.268012	0.156027

Notes

Naming convention (matches R fixest)::

i(cyl)            -> cyl::4, cyl::6, cyl::8
i(cyl, ref=4)     -> cyl::6, cyl::8
i(cyl, wt)        -> cyl::4:wt, cyl::6:wt, cyl::8:wt
i(cyl, wt, ref=4) -> cyl::6:wt, cyl::8:wt