Cost analysis
In the non-conditional incentive arm, there were 220 participants recruited, of these 215 received a £20 voucher despite only receiving 127 diaries back. Therefore the total cost for the vouchers was £4300 and for each questionnaire actually returned the cost was £33.85. In the conditional incentive arm, because vouchers were only sent out once a diary was received, the total cost for the vouchers was £1820 and the cost for each diary received is £20.00.For the indirect costs in the non-conditional arm with 215 vouchers distributed, the total cost for the administration time was £1433 and £11 per diary received. For the conditional arm, the total cost of the administration was roughly £159 and £1.75 per diary received. The total additional cost of the second class letter was £50.96 making a total of roughly £2.31 per diary received when considering administration and postage.Therefore, the total cost for the non-conditional incentive arm was £5733 and per diary received was estimated to be £45. For the conditional incentive arm, the total was estimated to be around £2029 with a per diary cost of £22 a total difference of £23 in favour of the conditional arm.