Inference: Support `fixef_k = "nested"` for small sample correction in `ssc()` for identical standard errors with `fixest`

[s3alfisc]

Context

At the moment, PyFixest's standard errors differ slightly from fixest defaults (explainer here).

This is because fixef_k = "nested" is not implemented in PyFixest - in contrast to fixest, the default argument of the ssc() function is fixef_k = "none" instead of "nested".

Per the fixest docs, "fixef.K="nested" discards all coefficients of fixed effects nested within clusters" when computing the number of coefficients $k$ for small sample corrections, while "fixef.K="none" discards all fixed-effects coefficients".

For examples of what is meant by a fixed effect nested in a cluster, take a look at the fixest vignette.

Task

Add the following variants to the fixef_k function argument of the

pyfixest/pyfixest/utils/utils.py

Line 11 in a98a9b9

def ssc(

function:

• fixef_k="nested"
• fixef_k="full"

Add unit tests against fixest.

Change the default to ssc(fixef_k = "nested"), mirroring fixest.

Note that this introduces a breaking change to the PyFixest results - all default standard errors will deviate slightly.

[valak70]

Hi @s3alfisc, I'm not familiar with fixest but what I understood is replacing fixef_k: str = "none" to fixef_k: str = "nested" will fix this