A middle class image of society: a study of undercoverage and nonresponse bias in a telephone survey

We studied undercoverage and nonresponse in a telephone survey among the population of the City ofGroningen, the Netherlands. The original sample, drawn from the municipal population register,contained 7000 individuals. For 37 percent of them, the telephone company was unable to produce a validtelephone number. Of those with a known telephone number, 49 percent did not answer the telephone orrefused to cooperate. As a result, the final respondents comprised merely 32 percent of the originalsample. To study distributional bias, we used individual-level data compiled from municipal records asour benchmark. Bivariate as well as multivariate analyses showed the undercoverage to be stronglyrelated to all sociodemographic variables studied, except gender. Nonresponse was related to age, countryof origin, marital status, and household size and composition, but not to gender, unemployment, socialassistance benefit, and education. Both undercoverage and nonresponse contributed to a strong middleclass bias in the final data set: middle-aged and older, economically secure people, of Dutch origin andliving with others in a household are overrepresented, while persons in disadvantaged and marginalpositions, such as the young, people of foreign stock, the unemployed, persons depending on publicincome support and singles are underrepresented.