Abstract
Aim: High participation is important for longitudinal studies to reduce bias. We evaluated whether a small monetary incentive (gift or 'koha' in Māori) increased consent and questionnaire completion rates, and decreased time to consent in the GEMS Follow-up Study.
Methods: We conducted a before and after comparison during follow-up at 4.5 years of the GEMS randomised trial of diagnostic thresholds for gestational diabetes. Participants were invited to consent and complete health questionnaires for themselves and their child. The intervention (koha) was a NZ$25 supermarket voucher. Consent and questionnaire completions were compared between Koha and No Koha groups using Chi-square. Times to these outcomes were compared using survival analysis.
Results: The Koha (n = 838) and No Koha groups (n = 368) had similar baseline characteristics. Consent rates were similar in Koha and No Koha groups for mothers (72.8% vs. 70.4%, p = 0.390) and children (69.1 vs. 69.0%, p = 0.980). Questionnaire completion was also similar for mothers (71.6 vs. 68.2%, p = 0.234) and for children (67.6% vs. 66.6%, p = 0.711), although the Koha group were more likely to complete the full child questionnaire (p = 0.019). Median times to maternal and child consent were also similar between groups for those who consented (24 vs. 24 days and 23 vs. 22 days), but survival analysis showed time to consent was reduced in the Koha group for mothers (p < 0.001) and children (p = 0.001). Findings did not vary by maternal ethnicity.
Conclusions: Introduction of a small monetary incentive (koha) did not increase consent or questionnaire completion rates, but was associated with reduced time to consent.