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Table 3 Results of the analysis

From: The effect of two lottery-style incentives on response rates to postal questionnaires in a prospective cohort study in preschool children at high risk of asthma: a randomized trial

Crude effects of intervention on outcome
Intervention Daytrip
Outcome RD 95% CI NNT
All questionnaires returned (100% returned) −0.008 −0.07 – 0.06 126 (NNH*)
All questionnaires returned without reminder 0.019 −0.03 – 0.07 51
Non-response 0.0004 −0.03 – 0.03 2,359
Withdrawal 0.05 −0.01 – 0.09 19
Outcome Median-difference intervention group-control group p-value of ranksum-test  
Number of reminders sent 0.5 0.753  
Percentage of all questionnaires returned 0 0.803  
Intervention Gift voucher
Outcome RD 95% CI NNT
All questionnaires returned (100% returned) 0.017 −0.50 – 0.08 60
All questionnaires returned without reminder 0.015 −0.04 – 0.07 67
Non-response −0.017 −0.05 – 0.02 60 (NNH*)
Withdrawal 0.008 −0.03 – 0.05 118
Outcome Median-difference intervention group-control group p-value of ranksum-test  
Number of reminders sent 1 0.045  
Percentage of all questionnaires returned 0 0.374  
Outcome Odds Ratio p-value of multilevel logistic regression 95% CI
Response Rate per questionnaire 1.29 0.26 0.83 – 2.01
  1. Table 3 presents the results of the analysis on “all questionnaires returned, “without reminders” versus “not all questionnaires returned or reminders sent”. Risk Differences (RD) with 95% confidence intervals and Number Needed Treat (NNT = reciprocal of RD) for are presented.
  2. CI 95%-level of NNT is not given, because all outcomes are not significant. In case of a non-significant treatment effect at 5%, the CI95% for the risk difference will include zero, and thus the 95% confidence interval for the number needed to treat will include infinity (∞). For example the effect of daytrip on non-responders gives an RD of 0.0004 (CI95% -0.03 – 0.03; including zero). The NNT is 2359 (1/0.0004), if we calculate CI95% on the same way, it must include infinity, thus from 32 to ∞ and from minus 31 to minus ∞ [6].
  3. * Number needed to harm (NNH in case of a negative risk difference).