As an example, see ‘The Effects of Transparent Outgoing Envelopes on the Response Rate and Speed in Mail Surveys‘ by Hiromitsu Maeda and Shingo Abe (Volume 7, No. 1, June 2010).
“There are few studies that have examined the influences of transparent or see-through envelopes…”
“In this study, a mail survey was conducted in order to examine the effects of transparent envelopes (those allowing visualization of contents) on response rate and speed. The experiment was carried out by mailing a questionnaire covered with either transparent outgoing envelope or plain one to 1,000 households, whom were chosen by two-stage area sampling.”
“The results of this experimental mail survey indicated that the use of transparent outgoing envelopes did not significantly stimulate the survey response.”