The prominent National Longitudinal Lesbian Family Study, for instance, relied on a sample recruited entirely at lesbian events, in women's bookstores and through lesbian newspapers.

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Although sampling can also lead to errors if, for instance, the characteristics of the sample group differ from those of the population from which the sample is drawn the errors will be smaller and can be measured more precisely.

ECONOMIST: Statistics

But the relevant sample size for many purposes for instance, measuring the uncertainty of the analysis is 33, the number of clusters.

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Mr Heckman confronted the sample-selection problem: in terms of the examples just given, for instance, the effect on the results if the workers who answered the questionnaire differed in important ways from those who did not.

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