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Table 2 Primary and secondary outcomes in the SWAT

From: Evaluation of the effectiveness of an incentive strategy on the questionnaire response rate in parents of premature babies: a randomised controlled Study Within A Trial (SWAT) nested within SIFT

  Before group
(n = 459)
After group
(n = 464)
Unadjusted effect measure (95% CI)a p value Adjusted effect measure (95% CI)ab p value
Questionnaire received (at 2 years of age corrected for prematurity)c, n/N (%) 381/459 (83.0) 353/464 (76.1) 6.9 (1.7, 12.1) 0.009 6.8 (1.6, 12.0) 0.010
Method of completion:       
 Paper questionnaire, n/N (%) 326/459 (71.0) 295/464 (63.6)   0.061f   
 Online completion, n/N (%) 50/459 (10.9) 53/464 (11.4)     
 Completion via telephone, n/N (%) 5/459 (1.1) 5/464 (1.1)     
Total cost of the strategyd (£)       
 Mean (SD) 17.97 (1.7) 15.00 (6.7) 2.97 (2.33, 3.60) < 0.001 2.98 (2.34, 3.61) < 0.001
 Median (IQR) 18.22 (17.41 to 18.40) 18.08 (18.08 to 18.88)     
Increase in cost compared to prior to commencement of incentives study (£, 95% CI) 14.80 (14.68, 14.94) 11.84 (11.51, 12.12)     
Reminder requirede, n/N (%) 248/456 (54.4) 264/452 (58.4)     
 0 208/456 (45.6) 188/452 (41.6)     
 1 104/456 (22.8) 95/452 (21.0)   0.027g   
 2 43/456 (9.4) 31/452 (6.9)     
 3 101/456 (22.1) 138/ 452 (30.5)     
 Missing 3 12     
  1. aAbsolute difference in the proportion responding and mean difference for cost
  2. bAdjusted for stratification factors: trial allocation and single or multiple birth
  3. cReceipt of a completed or partially completed (defined as completion of the first three sections or more) questionnaire at the SIFT office (note that the denominator is the number of eligible infants, which takes into account multiple births)
  4. dCosts included postage, receipt of material via Freepost envelope, envelopes, sticker sets, and value of gift vouchers but excluded costs of Freepost licence fee, printing, telephone calls, and trial staff time
  5. eReminder 1 sent by post; reminder 2 sent by post plus accompanying phone call; reminder 3 phone call only
  6. fp value for chi-squared test
  7. gp value for chi-squared test for trend (linear association)
  8. SD denotes Standard deviation; Interquartile Range (IQR) denotes 25th to 75th percentiles