The author compares aspects of voluntary and involuntary sample surveys in West Germany. "The German microcensus as a non-voluntary survey draws a random sample from the total population which includes persons that would also respond in a voluntary survey (respondents) and persons that would not respond (non-respondents). The population of a voluntary survey, however, includes only respondents. Hence, statistical inference from a voluntary sample survey is only valid for the total population, if the population of respondents does not differ from the total population. This null hypothesis must be rejected from the comparisons of data from the German microcensus of 1985, 1986 and 1987 and corresponding voluntary test sample surveys. The discrepancies are great in central demographic and socio-economic variables such as region of residence, community size, age, marital status, income and social security." (SUMMARY IN ENG)