program define _crcsrv2
version 2.1
	mac def _varlist "req ex"
	mac def _if "opt"
	mac def _in "opt"
	parse "%_*"
	parse "%_varlist", parse(" ")
	cap drop _xi
	cap drop _surv
	cap drop _stds
	cap drop _vlogs
        cap drop _atrisk
	capture {
		if "%_3"!="" {
			mac def _byc "by %_3:"
		}
		gen _stds = - %_2
		sort %_3 %_1 _stds /* deaths first */
		gen int _xi = 0
		%_byc replace _xi = 1 %_if %_in
		%_byc replace _xi = 0 if %_1==. | %_2==.
		%_byc replace _xi = sum(_xi)
		%_byc replace _xi = _xi[_N] + 1 - _xi
		gen _surv = 1
		replace _surv = (_xi - %_2) / _xi %_if %_in
		replace _surv = 1 if %_1==. | %_2==.
		%_byc replace _surv = _surv * _surv[_n-1] if _n>1
		by %_3 %_1: replace _surv = _surv[_N]
		label var _surv "Survival Probability"
		replace _stds = %_2 / _xi / (_xi - 1)
                gen int _atrisk=_xi-1
		replace _xi=-1 %if %in /* -1 ==> in the sample */
		replace _xi=0 if %_1==. | %_2==.
		replace _stds=0 if _xi!=-1
		%_byc gen _vlogs = sum(_stds)
		by %_3 %_1: replace _vlogs = _vlogs[_N]
		replace _surv=. if _xi!=-1
		replace _vlogs=. if _xi!=-1
		replace _stds = _surv*sqrt(_vlogs)
		label var _stds "Greenwood Survival S.D."
		label var _vlogs "Var(log(_surv))"
                label var _atrisk "N at risk at t+"
                drop _xi
	}
	mac def _rc = _rc
	exit %_rc
end