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.

WSJ: Nelson Lund: A Social Experiment Without Science Behind It

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.

ECONOMIST: Economics focus