*! version 2.00 14/06/99  (STB-53: sg128)
program define gullplot
   version 6.0
*First written 16/09/98
*Author: Salgado-Ugarte I.H.
*This program estimates the Linfinite and K parameters of the von
*Bertalanffy growth function by means of the Gulland-Holt method
*Gulland and Holt (1959) adjusted by Gulland (1969) using mean 
*length at age values.
*This new version is updated for Stata 6.0

   #delimit ;
   syntax varlist(min=1 max=1) [if] [in]
   , [noGraph T1title(string) Symbol(string) Connect(string) *];

   #delimit cr
   tokenize `varlist'

quietly {
preserve

tempvar msize
gen `msize' = `1' `if' `in'
*drop if `msize'==.
*if `msize'[1]==. {
*   di in red "no observations"
*   exit}
*keep `msize'
local nurows=_N+2
set obs `nurows'
tempvar lagmsi lmsize llagm delsi delest b
gen lagmsi=`msize'[_n+1]
gen lmsize= `msize'[_n-1]
gen llagm= lagmsi[_n-1]
gen delsi= llagm-lmsize

replace lmsize=0 if _n==1

regress delsi lmsize
predict `delest'

local b=_b[lmsize]
 if `b'>=0 {
    noi di in red "The slope of the line of the observed points is >=0" 
    exit}
 else 

local kval=log(_b[lmsize]+1)*-1
local linf=_b[_cons]/(1-exp(`kval'*-1))

replace `delest'=0 if _n==_N
replace lmsize=`linf' if _n==_N

noi di _newline "Estimation of L_inf and K values by the Gulland Method"
noi di _dup(60) "-"
noi di "Intercept = " %8.4f _b[_cons] _skip(10) "Slope         = " %8.4f _b[lmsize]
noi di _newline "R-squared = " %8.4f _result(7) _skip(10) "Adj R-squared = " %8.4f _result(8) 
noi di _newline "L_inf.    = " %8.4f `linf' _skip(10) "K             = " %8.4f `kval'
noi di _dup(60) "-"

    if "`graph'" ~= "nograph"  {
       local lilab=string(round(`linf',.0001),"%9.4f")
       local kvlab=string(round(`kval',.0001),"%9.4f")
       if "`t1title'" ==""{
          local t1title "Gulland graph"
          local t1title "`t1title',  L_inf = `lilab',  K = `kvlab'"
           }
       if "`symbol'"=="" { local symbol "O." }
       if "`connect'"=="" { local connect ".l" }
       if "`l1lab'"=="" {local l1lab "Delta Size" }
       label var lmsize "Size at age t"
       graph delsi `delest' lmsize, `options' /*
				*/ t1("`t1title'") /*
				*/ s(`symbol') c(`connect') /*
                                */ l1(`l1lab')
       } 
}
end