Table 1

Response rates by study group and multivariate analysis

Group Control (n=374) Lottery (n=374) Pooled non-payment (n=748) Cost per sampled woman
(cost per response)
Payment (n=125) Lottery and payment (n=125) Pooled payment (n=250) Cost per sampled woman
(cost per response)§
Marginal cost per completed response of payment compared with non-payment
Response rate
Initial mailing37.7% (141/374)41.7% (156/374)39.7%45.6% (57/125)49.6% (62/125)1-150 47.6%7.9%
£1.70 (£4.28)£3.50(£7.35)£1.80 (£22.78)
1st reminder20.4% (38/186)1-150 21.0% (39/186)1-150 20.7%22.0% (13/59)1-150 31.7% (19/60)1-150 26.9%6.2%
£1.70 (£8.21)£3.05 (£11.34)£1.35 (£21.77)
2nd reminder17.6% (22/125)1-150 18.8% (24/128)1-150 18.2%29.6% (13/44)1-150 14.3% (5/35)1-150 22.8%4.6%
£1.70 (£9.34)£2.84 (£12.46)£1.14 (£24.78)
Total53.7% (201/374)58.6% (219/374)56.1%66.4% (83/125)68.8% (86/125)67.6%11.5%
£3.12 (£5.56)£5.85 (£8.65)£2.73 (£23.74)
Logistic regression analysis modelling of response by incentive method
Variable Coefficient (β) Odds ratios (95% CI) χ2 1 p Value
Payment0.531.70 (1.11 to 2.60)6.230.013
Lottery0.201.22 (0.91 to 1.62)1.760.185
Payment-lottery interaction−0.090.92 (0.50 to 1.68)0.080.780
Constant0.15
  • 1-150 Subjects returning a blank form received no further reminders so that the denominator for reminder response rates are smaller than the numbers not responding to the previous mailing. Assumes 29 women did not accept payment therefore costs are based on payment to 90 women not 119. The cost per completed response in the pooled non-payment column was calculated by dividing the cost of the questionnaire (£1.70) by the proportion responding, for example, £1.70 / 0.397 = £4.28. The total cost per sampled woman was the total cost of questionnaires divided by women sampled (£2334.10 / 748) and the final cost per respondent by dividing the total cost by the number of responders (£2334.10 / 420). §The cost per sampled woman was calculated by adding the total cost of questionnaires sent to the cost of 90 respondents who accepted payment divided by the number of women sampled, for example, (£1.70 × 250) + (£5 × 90) / 250. The cost per response was calculated by dividing the cost by the number of respondents, for example, (£1.70 × 250) + (£5 × 90) / 119. The total cost per sampled woman was the total cost of questionnaires divided by women sampled (£1461.60 / 250) and the final cost per respondent was the total cost divided by the number of responders (£1461.60 / 169). The marginal cost per completed response of payment compared with non-payment was calculated from the difference in response rates and costs from the pooled non-payment and pooled payment columns, for example, the total marginal cost was (£5.85−£3.12) / (0.676–0.561).