*! overlap -- Calculate the overlapping coefficient     (sg27: STB-22)
*! version 1.1.0   14 August 1993  R. Goldstein
program define overlap
    version 3.0
    local varlist "req ex max(2)"
    local if "opt"
    local in "opt"
    local options "BY(string)"
    parse "`*'"
    parse "`varlist'", parse(" ")
if "`by'"=="" {
    qui su `1' `if' `in'
    local numb1=_result(1)
    local mean1=_result(3)
    local sd1=sqrt(_result(4))
    qui su `2' `if' `in'
    local numb2=_result(1)
    local mean2=_result(3)
    local sd2=sqrt(_result(4))
overlapi `mean1' `mean2' `sd1' `sd2' `numb1' `numb2'
    exit
}
    confirm var `by'
    quietly tab `by' `if' `in'
    if _result(2)!=2 {
    di in red "`by' takes on " _result(2) " values, not 2"
    exit 450
    }
        tempvar M TOUSE
        quietly gen byte `TOUSE' = 1 `if' `in'
        sort `by' `TOUSE'
        quietly by `by' `TOUSE': gen byte `M'=cond(_n==1,1,.) if `TOUSE'==1
        sort `M' `by'
        quietly sum `varlist' if `by'==`by'[1] & `TOUSE'==1
    local numb1=_result(1)
    local mean1=_result(3)
    local sd1=sqrt(_result(4))
        quietly sum `varlist' if `by'==`by'[2] & `TOUSE'==1
    local numb2=_result(1)
    local mean2=_result(3)
    local sd2=sqrt(_result(4))
overlapi `mean1' `mean2' `sd1' `sd2' `numb1' `numb2'
end