The Swiss Hidden Population Study: practical and methodological aspects of data collection by privileged access interviewers

In order to recruit heroin and/or cocaine users outside treatment settings, recruitment of subjects through Privileged Access Interviewers (PAI) was tested and implemented in the Swiss Hidden Population Study. This article discusses practical aspects of the PAI method as well as issues of reliability and validity. From June 1994 to June 1995, 31 Privileged Access Interviewers were recruited in the main regions of Switzerland. They conducted 943 standardized interviews altogether, of which 917 could be considered valid. Fifty-four per cent of the respondents correspond to the criteria of the target population. The PAI method collects reliable data in a relatively short amount of time, given adequate means of control. Analysis of the age distribution and of the patterns of drug use in our sample shows that the question of validity is mainly linked to the diversity of the milieus from which PAIs recruit the respondents. Encouraging PAIs to do as many interviews as possible did not skew the data. Hence, well-founded inferences on a PAI generated database relies on the analysis of qualitative information on the ways in which the Privileged Access Interviewers have recruited their respondents.