A small envelope enclosed with a mailing in which, for example, a reply card can be returned. To maximise response rates, it should be easy for the respondent to respond to a mailing. If stamps have to be put on a return envelope, it costs the respondent not only effort but also money. Therefore, a reply number is pre-printed on the return envelope. The respondent can then send the return envelope for free, without having to affix stamps.
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