In some countries, additional sample surveys are carried out immediately after a 100 per cent census of the population, and using the census returns as the sampling frame to identify a random sample of the total (adult) population or else sub-groups within it (such as all persons with higher education qualifications). Response to follow-up surveys is usually voluntary whereas most countries ensure that participation in the population census is compulsory. Follow-up surveys may be postal surveys, if large samples are involved, or personal interview surveys, if they focus on smaller sub-groups. Follow-up surveys provide substantive additional information to that collected in the census, unlike post-enumeration surveys. See Catherine Hakim, Secondary Analysis in Social Research (1982).