00346nam a2200109 a 450000500170000010000330001724500750005026000440012530000220016963000250019163000200021620251203092055.0  aWorld Health Organisationd_  aSampling methods in morbidity surveys and public health investigations  aGenevabWorld Health Organisationc1966  a29 p.c26 cm.bPB  aResearch methodology  aSocial research