A controlled trial of envelope colour for increasing response rates in older women

Postal questionnaires are widely used in health research to provide measurable outcomes in areas such as quality of life. Participants who fail to return postal questionnaires can introduce non-response bias. Previous studies within populations over the age of 65 years have shown that response rates amongst older people can be 60% or less. The current study sought to investigate whether envelope colour affected response rates in a study about the effectiveness of screening older women for osteoporosis.

A total of 2803 eligible female participants aged between 70 and 85 were sent an invitation pack from their GP practice. The invitation was either in a brown or white envelope and contained a matching pre-paid reply envelope. A study questionnaire was also sent out in brown or white envelopes 1 week after consenting to participate in the trial.

The overall response rate was 78%. There was little evidence of an effect of envelope colour on response to the invitation to participate in the trial (OR 1.04, 95% CI 0.87-1.24). Similarly, there was no influence of envelope colour on the number of participants returning their questionnaires (OR 0.99, 95% CI 0.60-1.63). There was weak evidence of an effect of envelope colour on the response rates of the consent process (OR 0.86, 95% CI 0.74-1.00). When we updated a recent meta-analysis with the results of this study, there was a non-statistically- significant trend for greater response rates with brown envelopes compared with white envelopes (OR 1.19, 95% CI 0.86-1.64, I2=92%). However, the results where influenced by one study and when this study was excluded the pooled estimate was 0.98 (95% CI 0.89-1.08, I2=0%).

This study found no evidence to suggest envelope colour has an effect on response to participate in a trial or questionnaire returns. There is weak evidence to suggest envelope colour may affect consent into a trial.