“…The sampling techniques of the selected papers include representative population samples 10 , 14 , 21 , 22 , 25 , 29 , 46 , Respondent-driven sampling (RDS) 15 , 26 , 45 , convenience sampling 23 , 43 , 47 , 50 , 65 , birth cohort 54 , 53 and multistage conglomerate survey 49 . Eight other studies did not collect primary data, using secondary data or testimony from other research 16 , 23 , 28 , 30 , 31 , 48 , 51 , 59 .…”