* Calculation of median age at first sex

set more off

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program define calc_median_age

summarize age [fweight=v005] if v012>=25 & v012<=49, detail

scalar sp50=r(p50)

gen dummy=.
replace dummy=0 if v012>=25 & v012<=49
replace dummy=1 if v012>=25 & v012<=49 & age<sp50
summarize dummy [fweight=v005]
scalar sL=r(mean)

replace dummy=.
replace dummy=0 if v012>=25 & v012<=49
replace dummy=1 if v012>=25 & v012<=49 & age<=sp50
summarize dummy [fweight=v005]
scalar sU=r(mean)
drop dummy

scalar smedian=round(sp50+(.5-sL)/(sU-sL),.01)
scalar list sp50 sL sU smedian

* warning if sL and sU are miscalculated
if sL>.5 | sU<.5 {
ERROR IN CALCULATION OF L AND/OR U
}

drop age
end

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* EXECUTION BEGINS HERE

* sp50 is the integer-valued median produced by summarize, detail;
*   what we need is an interpolated or fractional value of the median.

* In the program, "age" is reset as age at first cohabitation or age at first birth;
*   with modifications, other possibilities would require modifications.

* sL and sU are the cumulative values of the distribution that straddle the integer-valued median

* v011 date of woman's birth (cmc)
* v211 date of first child's birth (cmc)
* v511 age at first cohabitation 

set maxvar 10000
use e:\DHS\DHS_data\IR_files\PHIR61FL.dta, clear


* age at first sex calculated from v531
gen afs=v531
replace afs=99 if v531==. | v531==0
replace afs=. if v531==98 | v531==99
gen age=afs
calc_median_age
scalar safs_median=smedian
scalar list safs_median