Dear colleagues, good day! I would be very grateful if you could help me with an issue.

Setup: I have data on siblings, 2-3 children per family. Totally, there are about 4000 families. Dependent variable is "Educational attainment" (continuous), independent variable is "Cognitive ability" (continuous). I am running family fixed effects (xtreg) to take into account any confounding effects shared by siblings.

Questions:
1. Which N should I report in my paper? The nominal N that STATA reports? Or N with only discordant sibling families (where, for example, all the children have different educational attainment and cognitive ability)? For example, xtlogit just drops those groups (families) where individuals do not differ in terms of a dependent variable (thus they are non-discordant) and reports only N with non-discordant families. But obviously, xtreg does not work in that way, because it always reports the nominal N.

2. Which families are really involved in calculating b-coefficients and standard errors? All the cases despite whether or not the children are discordant? Only those that differ within-family in terms of the depvar? Or only those that differ in terms of the indepvar? Or only when both depvar and indep var are discordant?

With best regards,
V