evidence: The effect of the voucher intervention on ‘percentage of all questionnaires received’ was not significant (Wilcoxon P value 0.37).
intervention: (i) daytrip tickets for the whole family to a popular amusement park if they returned all postal questionnaires, (ii) €12.50-worth gift vouchers for sending back the questionnaire on time after each questionnaire round and (iii) a combination of (i) and (ii)
comparator: contol
outcome: the proportion of participants who returned all questionnaires