Hello everyone! So a little context is that the last statistics module I had was in my bachelor's all the way back in 2015. I'm currently a PhD candidate and I'm trying to expand my methodological horizons into less qualitative approaches. Also, english is not my first language, so apologies if I'm a little too verbose :)

Recently I've been looking at econometrics and regression models to estimate institutional impacts on things I'm interested in writting about within social sciences through panel data.

The papers I've read (some stuff from Bueno Mesquita's Selectorate Theory comes to mind) usually use region-year fixed effect to - as I understand it - estimate within-regional effects of variance of specific institutional variables compared to the average development within that region-year scope and then move on to robustness check with country fixed effects to check the results against within country variation for the years under analysis.

My question essentially is the following: how do I know how many fixed-effects are enough fixed effects? any literature recomendations? Should I, for instance on a panel with 170 countries over 4 years, have a test for region-year FE checked for robustness against a test for country FE? Would that be "enough"? Or should I then run another check for FE with, say, country+year or Region-Year + Country? Or would that just "squish out" any interesting observations?

Essentially, how much control is too much control and at which point can I go "oh yes, this funny number means interesting things"?

Thank you all! <3