Snowball sampling by mail: application to a survey of smokers in the general population

Background In a series of surveys intended for current and former smokers b ut sent to a random sample of the general population, we asked never smoker s and smokers who did not wish to participate to transmit the questionnaire to any ever smoker they knew. We compared participants who received the qu estionnaire directly from us (original participants) to participants who re ceived it from an addressee (secondary participants). Methods Questionnaires on smoking were mailed to 3300 residents of Geneva ( Switzerland) in 1997, and returned by 1167 people (35%). Results The final sample consisted of similar numbers of original participa nts (n = 578, primary response rate = 18% of total sample, or about 46% of ever smokers) and secondary participants (n = 566). Original participants w ere 1.7 years older than secondary participants (P = 0.03) and were more li kely to be men (50% versus 43%, P = 0.009). Proportions of current smokers, stages of change, confidence in ability to quit smoking, cigarettes per da y and attempts to quit smoking were similar in the two groups. Secondary pa rticipants had lower self-efficacy scores (-0.30 standard deviation (SD) un its, P less than or equal to 0.03), and they derived more pleasure from smo king (+0.25 SD units, P = 0.04). Among ex-smokers, direct participants were less active than secondary participants in coping with the temptation to s moke (-0.58 SD units, P = 0.002). Associations between smoking-related vari ables were similar in original and secondary participants. Conclusions Allowing non-eligible addressees to transmit the questionnaire to someone else doubled the response rare, produced moderate bias on some v ariables only and had no detectable impact on associations between smoking- related variables.