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Table 2 Duration between mailing date of the invitation and active response

From: Study invitations with envelopes made from recycled paper do not increase likelihood of active responses or study participation in the German National Cohort

  Mean sd ßa,b (95% CI)
Envelope color
 White 7.5 3.1 0
 Grey 7.4 3.2 0.65 (− 0.38, 1.68)
Sexc
 Female 7.4 3.0 0
 Male 7.4 3.3 0.06 (− 0.92, 1.04)
Nationalityc
 Non-German 8.2 3.2 0
 German 7.4 3.2 − 0.56 (− 2.59, 1.47)
Age
 20–29 7.4 2.9 − 0.53 (− 2.45, 1.39)
 30–39 6.8 2.8 − 0.66 (− 3.4, 2.08)
 40–49 7.5 2.8 0
 50–59 8.0 3.6 0.53 (− 0.71, 1.77)
 60–69 6.8 3.1 − 0.74 (− 2.03, 0.54)
N 171   
  1. CI confidence interval
  2. aIncludes subjects who replied by letter, phone call, e-mail or visited the study center in person
  3. bAdjusted for pre-trial likelihood of active responses to invitation letter stratified by weekdays
  4. cInformation provided by Bremen’s resident registration office