Table 3
Baseline Characteristics of sites
| Non-Conditional Incentive Arm (n = 21) | Conditional Incentive Arm (n = 21) |
Previous Recruitment – mean (range) (for those recruited in Season 1) | 5 (1–19), n = 9 | 4(1–14), n = 9 |
Practice List Size – median (IQR) | 9674 (7900,15408) | 12055 (7050,15600) |
Table 4
Baseline Characteristics of participants
| Non-Conditional Incentive Arm (n = 220 unless otherwise stated) | Conditional Incentive Arm (n = 125 unless otherwise stated) |
Duration of Illness (days) – median (IQR) | 2 (2,3) | 3 (2,3) |
Score of clinical Severity – median (IQR) | 2 (1,2) | 2 (2,2) |
Age – median (IQR) | 26 (12,46) | 36 (23,53) |
Temperature – median (IQR) | 37.1 (36.6, 37.6) | 37.6 (37.0,38.2), n = 124 |
Female – n (%) | 135 (61) | 73 (58), n = 124 |
The baseline characteristics of previous recruitment was relatively well balanced though practice size was not (Table 3). The baseline characteristics of the participants within the SWAT were well balanced (Table 4). The baseline participant level data was reasonably well balanced between intervention groups.
Of the 42 sites involved in the study, only twenty-eight (67%) sites recruited at least one participant. Only 10 of the 21 (48%) site pairs had at least participant recruited in both sites within the pair. The median (IQR) numbers recruited across all sites was 2 (0,10), and by conditional and non-conditional arms were 3 (0,10) and 3 (0,10) respectively.
Compliance
All participants in the conditional incentive group received their vouchers. Within non-conditional group there were 5 participants who did not receive the vouchers as they should have done. This was due to site non-compliance and non-response to the trial team who were trying to restock their vouchers. The site were re-trained and the issue did not occur again.
Table 5
Overall Recruitment and outcomes by Intervention Group at the participant level
| Participants recruited across sites | Diaries received across sites – n (%) | Days to diary return – median (IQR) | Number of pages completed – median (IQR) |
Non-Conditional Incentive | 220 | 127 (58) | 23 (19, 27) | 35 (35, 35) |
Conditional Incentive | 125 | 91 (73) | 20 (18, 25) | 35 (35, 35) |
Total | 345 | 218 (63) | 21 (18, 27) | 35 (35, 35) |
Return of Diaries
Twenty six of the twenty eight (93%) sites that recruited at least one participant returned one or more diaries; the proportion returned varied from 0 to 1.0 within both intervention groups reflecting the small number recruited per site. Median and IQR site proportions returned were 0.66 (0.57, 1.00) and 0.71 (0.50, 0.80) for the non-conditional and conditional incentive groups respectively. The raw proportion returned irrespective of site was 0.58 and 0.73 for non-conditional and conditional incentive groups. Corresponding values for the subset (10 pairs) where both of the constituent sites paired at randomised recruited one or more participant were 0.56 and 0.73. The median and IQR for the aforementioned subset of 10 pairs was 0.64 (0.57, 1.00) and 0.71 (0.50, 0.92) for non-conditional and conditional incentive group respectively.
Regression analysis adjusted for cluster pair was a difference between interventions of -0.09, 95% CI (-0.29, 0.10), p = 0.336. The sensitivity analysis which also weighted by the number of individuals recruited at each site produced a substantial different raw effect though it was still not statistically significant: 0.15, 95% CI (-0.02, 0.31), p = 0.068. Non-parametric test Wilcoxon signed rank test gave a similar finding of no clear statistical evidence of a difference (p = 0.760). The other sensitivity analyses which ignored the pairs and analysed all sites where one or more participant was recruited also gave similar findings with Mann-Whitney p = 0.695, and unweighted paired t test had a mean difference of -0.04, 95% CI (-0.28, 0.19), p = 0.701.
Time to Diary Return
The individual time to return ranged from 10 to 64 days across both intervention groups. Median and IQR of the average times to return by intervention group at site level were 23 (20, 26) days and 21 (19, 25) days for non-conditional and conditional incentive group respectively. Corresponding values for the subset (9 pairs) where both constituent sites had at least one diary returned were 24 (23, 25) days and 21 (19, 24) days for non-conditional and conditional incentive group respectively.
Regression analysis adjusted for cluster pair was a mean difference between interventions of -0.9, 95% CI (-5.7, 3.8) days, p = 0.685. The sensitivity analysis which also weighted by the number of individuals recruited at each site produced a substantial different raw effect which was statistically significant: -3.1, 95% CI (-5.0, -1.1), p = 0.004. Non-parametric test Wilcoxon signed rank test gave a similar finding (p = 0.214). The other sensitivity analyses which ignored the pairs and analysed all sites where one or more participant was recruited also gave similar findings with Mann-Whitney p = 0.354, and unweighted paired t test had a mean difference of -0.8, 95% CI (-7.0, 5.4) days; p = 0.768.
Number of Pages Completed
The average number of pages completed per diary returned varied from 7 to 36 pages across both intervention groups. Median and IQR average pages completed by intervention group at site level were 35 (34, 35) pages and 35 (35, 35) pages for non-conditional and conditional incentive group respectively. Corresponding values for the subset (9 pairs) where both constituent sites had at least one diary returned were 35 (34, 35) pages and 35 (35, 35) pages for non-conditional and conditional incentive group respectively. When returned, the diary was generally fully completed or almost fully complete; only 7 (3%) across all participants failed to return at least 30 pages when diary was returned. By intervention group this was 4 (3%) and 3 (3%) respectively for non-conditional and conditional intervention groups.
Regression analysis adjusted for cluster pair was a mean difference between interventions of 0.5, 95% CI (-1.2, 2.3) pages, p = 0.519. The sensitivity analysis which also weighted by the number of diaries returned at each site had a similar result: 0.3, 95% CI (-0.7, 1.2), p = 0.552. Non-parametric test Wilcoxon signed rank test gave a similar finding (p = 0.152). The other sensitivity analyses which ignored the pairs and analysed all sites where one or more participant was recruited also gave similar findings with Mann-Whitney p = 0.176, and unweighted paired t test had a mean difference of 0.20, 95% CI (-0.7, 1.1) pages; p = 0.612.
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 £4,300 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 £1,820 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 £1,433 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 £5,733 and per diary received was estimated to be £45. For the conditional incentive arm the total was estimated to be around £2,029 with a per diary cost of £22 a total difference of £23 in favour of the conditional arm.