Research studies that collect subject data with mailed questionnaires create unique challenges for investigators attempting to protect the confidentiality of the information. To determine who has responded in a cross-sectional study (for bias determination), investigators should consider having subjects return questionnaires anonymously, along with a postcard (mailed in separately but at the same time) with the subject’s name on it indicating that he/she has responded. Where it is necessary to know which questionnaire belongs to which subject, ID numbers can be assigned to individuals, with the questionnaire listing only the coded ID number. Once questionnaires have been reviewed for complete responses, the list containing the identifiers can be destroyed. In longitudinal research, investigators should consider having the subject provide his/her own unique identifier that will be used on all future questionnaires (one the subject will remember over time, such as birth date and some combination of numbers from his/her driver’s license or other stable code number that he/she will be able to regenerate). This will provide a link from questionnaire to questionnaire, but the investigator will not know the identity of the subject.