*! version 1.0.1  8Mar93 Royston; STB-12 sg1.5
program define nlgom4
	version 3.0
	if "`1'"=="?" {
		mac def S_2 "4-parameter Gompertz function, $S_E_depv=b0+b1*exp(-exp(-b2*(`2'-b3)))"
		mac def S_1 "b0 b1 b2 b3"
/*
	Approximate initial values by estimating upper and lower asymptotes.
*/
		local fac 1.1	/* fudge factor for asymptotes */
		local exp "[$S_E_wgt $S_E_exp]"
		tempvar Y
		quietly {
			gen `Y' = $S_E_depv $S_E_if $S_E_in
			sum `Y' `exp'
			local min = _result(5)
			local max = _result(6)
			if `max'<0 	{ local b = `max'/`fac' }
			else 		{ local b = `max'*`fac' }
			if `min'<0 	{ local a = `min'*`fac' }
			else 		{ local a = `min'/`fac' }
		}
		replace `Y' = -log(-log((`Y'-`a')/`b'))
		reg `Y' `2' `exp'
		mac def b2 = _b[`2']
		mac def b3 = -_b[_cons]/$b2
		mac def b0 `a'
		mac def b1 = `b'-`a'
		exit
	}
	replace `1'=$b0+$b1*exp(-exp(-$b2*(`2'-$b3)))
end