Also called census quality checks, post-enumeration surveys are carried out in selected areas immediately after population censuses have attempted to obtain a 100 per cent count, or enumeration, of the population. They have two purposes, usually achieved through separate exercises. Census coverage checks assess whether the 100 per cent enumeration was in fact achieved, and provide statistical estimates of any under-enumeration in particular types of area (such as inner-city areas with highly mobile populations), or in particular social groups (such as ethnic minorities). Census quality (validation) checks assess the quality of responses to the census, to provide statistical estimates of the validity of responses, gross and net error rates due to questions and/or response codes being misunderstood or misapplied. As errors cancel each other out to some extent, net error rates are always lower than gross error rates. Post-enumeration surveys repeat the census exercise in selected areas, but with a higher input of resources and more experienced survey interviewers than can be applied in the full census, so as to measure the quality of the larger data collection. They are now routinely carried out after every census and are quite different from census follow-up surveys which are occasionally carried out in a few countries.