Table 2:
Strategies that enhance or reduce the response rate to mailed questionnaires22
| Strategy | No. of studies | No. of participants | Odds ratio (95% CI) | Heterogeneity* | |
|---|---|---|---|---|---|
| I2 value, % | p value | ||||
| Enhances response rate | |||||
| Monetary incentive | 94 | 160 004 | 1.87 (1.73–2.03) | 84 | < 0.001 |
| Recorded delivery | 15 | 18 931 | 1.76 (1.43–2.18) | 71 | < 0.001 |
| Teaser on envelope† | 1 | 190 | 3.08 (1.27–7.44) | NA | NA |
| More interesting topic | 3 | 2 711 | 2.00 (1.32–3.04) | 80 | 0.01 |
| Prenotification | 47 | 79 651 | 1.45 (1.29–1.63) | 89 | < 0.001 |
| Follow-up contact | 19 | 32 778 | 1.35 (1.18–1.55) | 76 | < 0.001 |
| Unconditional incentive | 24 | 27 569 | 1.61 (1.36–1.89) | 88 | < 0.001 |
| Shorter questionnaire | 56 | 60 119 | 1.64 (1.43–1.87) | 91 | < 0.001 |
| Second copy of questionnaire at follow-up | 11 | 8 619 | 1.46 (1.13–1.90) | 82 | < 0.001 |
| Mention of obligation to respond | 3 | 600 | 1.61(1.16–2.22) | 0 | 0.98 |
| University sponsorship | 14 | 21 628 | 1.32 (1.13–1.54) | 83 | < 0.001 |
| Nonmonetary incentive | 94 | 135 934 | 1.15 (1.08–1.22) | 79 | < 0.001 |
| Personalized questionnaire | 58 | 60 184 | 1.14 (1.07–1.22) | 63 | < 0.001 |
| Handwritten address | 7 | 5 091 | 1.25 (1.08–1.45) | 14 | 0.3 |
| Stamped return envelope‡ | 27 | 48 612 | 1.24 (1.14–1.35) | 69 | < 0.001 |
| Assurance of confidentiality | 1 | 25 000 | 1.33 (1.24–1.42) | NA | NA |
| First class outward mailing | 2 | 8 300 | 1.11 (1.02–1.21) | 0 | 0.8 |
| Reduces response rate | |||||
| Questions of sensitive nature | 10 | 21 393 | 0.94 (0.88–1.00) | 0 | 0.5 |
Note: CI = confidence interval, NA = not applicable.
The I2 statistic describes the percentage of total variance across studies that can be attributed to heterogeneity (differences within and between studies) rather than to chance.30 Typically, I2 statistic thresholds of 0% to 40%, 30% to 60%, 50% to 90% and > 75% represent between-study heterogeneity that might not be important or that might be moderate, substantial or considerable, respectively.31 A χ2 test is typically used to assess heterogeneity. Because this test has low power in most meta-analyses (e.g., when studies have few patients and studies are few in number), a significant result may indicate a problem with heterogeneity; however, a nonsignificant result may not represent the absence of heterogeneity. Consequently, a p value < 0.1, rather than < 0.05, is usually used to determine statistical significance.
Comment that implies participants may benefit from opening envelope.
Versus franked return envelope.