Many important cross-sectional surveys are repeated at regular or irregular intervals so that estimates of changes can be made at the aggregate or population level. Examples include monthly labor force surveys, retail trade surveys, television and radio ratings surveys, and political opinion polls. These surveys are designed to give good estimates for the current population and the changes or movements that have occurred since the last survey or previous surveys. Typically surveys are conducted on a monthly, quarterly, or annual basis, although other intervals are possible, such as daily or weekly in the case of TV ratings and opinion polls. Surveys may also be conducted at longer intervals, such as 3 years, or repeated on an irregular basis, but in all cases there will be ...
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