Derivation
variable: hipcuf [first hip result] (table[s]: ncdk_persexam & ncdl_persexam)
variable: hipcus [second hip result] (table[s]: ncdk_persexam & ncdl_persexam)
Missing values removed and an average of the 2 (hipcuf, hipcus) measurements calculated; if only one measurement was taken that is used.
replace hipcuf=. if hipcuf==999
replace hipcus=. if hipcus==999
replace hipcuf=hipcus if hipcuf==.
replace hipcus=hipcuf if hipcus==.
gen hip=(hipcuf+hipcus)/2