step,rule,n_before,n_after,n_removed,rationale
1,drop rows with missing school_id/treated/score,1000,1000,0,cannot be used in OLS
2,drop exact duplicate rows,1000,1000,0,duplicate records
3,drop duplicated student_id (keep first),1000,1000,0,student_id must be unique
4,treated must be 0/1,1000,1000,0,binary treatment
5,drop schools with non-constant treated,1000,1000,0,treatment must be constant within school