Differential effects of pre and post-payment on neurologists' response rates to a postal survey.

The difference in the response rate was highly significant between the three groups (χ2 = 15.8, df = 2, p = 0.0004), with the group sent the incentive responding 61% of the time, compared with 48% for those promised the incentive, and 43% for those without incentive (see Table 1 ).

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