Table 2:

Strategies that enhance or reduce the response rate to mailed questionnaires22

StrategyNo. of studiesNo. of participantsOdds ratio (95% CI)Heterogeneity*
I2 value, %p value
Enhances response rate
Monetary incentive94160 0041.87 (1.73–2.03)84< 0.001
Recorded delivery1518 9311.76 (1.43–2.18)71< 0.001
Teaser on envelope11903.08 (1.27–7.44)NANA
More interesting topic32 7112.00 (1.32–3.04)800.01
Prenotification4779 6511.45 (1.29–1.63)89< 0.001
Follow-up contact1932 7781.35 (1.18–1.55)76< 0.001
Unconditional incentive2427 5691.61 (1.36–1.89)88< 0.001
Shorter questionnaire5660 1191.64 (1.43–1.87)91< 0.001
Second copy of questionnaire at follow-up118 6191.46 (1.13–1.90)82< 0.001
Mention of obligation to respond36001.61(1.16–2.22)00.98
University sponsorship1421 6281.32 (1.13–1.54)83< 0.001
Nonmonetary incentive94135 9341.15 (1.08–1.22)79< 0.001
Personalized questionnaire5860 1841.14 (1.07–1.22)63< 0.001
Handwritten address75 0911.25 (1.08–1.45)140.3
Stamped return envelope2748 6121.24 (1.14–1.35)69< 0.001
Assurance of confidentiality125 0001.33 (1.24–1.42)NANA
First class outward mailing28 3001.11 (1.02–1.21)00.8
Reduces response rate
Questions of sensitive nature1021 3930.94 (0.88–1.00)00.5
  • 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.