Derivation
variable: birthdd (table[s]: gen_identity)
variable: birthmm (table[s]: gen_identity)
variable: birthyear (table[s]: gen_identity)
variable: birthest (table[s]: gen_identity)
Birth date is created from individual date elements on gen_identity: if day missing 15 is used, if month missing June is used, if only year estimate the mid-point of the estimate range is used