When finding the percentage of female participants for the homework (and finding percentages in data analysis in general), are missing values usually included or excluded?
For the homework, we would like you to give the percentage of females out of all participants, not just those with known sex/gender. For research, it really depends on the goal of your analysis - generally, the same is also true (missing values included) but if you define your question "out of those with known gender, x% are female" then you would exclude missing values.
Default for STATA is to not include missing values in the denominator. For example, in this data set of 10 individuals, 3 have missing values and the denominator is 7.
If you want to include missing values in the denominator use the [tab varname, missing] option.