The components of landline telephone survey coverage bias. The relative importance of no-phone and mobile-only populations

The continuously growing mobile-only population raises concerns regarding the representativeness of traditional landline telephone surveys. At this time, the mobile-only population differs significantly from general population, which leads to coverage bias when using fixed-line samples only for telephone surveys. However, in many European countries the mobile-only population is not the only source of coverage bias in telephone surveys. In addition, we have to consider coverage biases caused by considerable proportions of citizens without any telephone service. Since these two groups differ from the general population with respect to differential socio-demographic categories, in our view, the negative effects of mobile-only coverage error in traditional landline telephone surveys might in fact compensate—in part—for coverage bias caused by the no-phone population. To test this hypothesis of compensating coverage biases we calculated relative coverage biases caused by the mobile-only population and relative coverage biases caused by the no-phone population in 30 European countries for two socio-demographic variables in two points in time. Results are presented for four groups of countries that differ with respect to no-phone and mobile-only rates. Results suggest that—in general—mobile-only biases and no-phone biases do not compensate to a great extent, and thus the alarming mobile-only biases cannot be neglected when using telephone surveys in the estimation of population parameters. Nevertheless, there are several countries where the bias caused by the mobile-only population is far bigger than the joint bias caused by the mobile-only population and the no-phone population. This finding suggests that biases caused by the recent mobile-only population would be even more severe if the no-phone population did not exist.