Given a vector of variables of interest for students in a single school, extracts a single value for the school
a vector containing student-level observations for a school. If it is a factor it must contain only a single level.
If the input is numeric,
agg returns the mean; if the input is not numeric, an error will be thrown unless all values are the same, in which case the single unique value will be returned.
A single value of the same type as the input vector.
Luke Keele, Penn State University, firstname.lastname@example.org
Sam Pimentel, University of Pennsylvania, email@example.com