BIAS IN LIST-ASSISTED TELEPHONE SAMPLES

A number of researchers have suggested list-assisted sampling for the selection of telephone households to overcome some of the operational difficulties associated with the Mitofsky-Waksberg methods of random d igit dialing (RDD). An advantage of a list-assisted method of RDD is t hat an equal probability systematic sample of telephone numbers can be selected and the variances of estimates from such a sample are usuall y lower than from a clustered design like the Mitofsky-Waksberg method . The main disadvantage of the list-assisted method is that it exclude s some households from the sample, thus creating a coverage bias in th e estimates. This article describes research on the coverage bias for a particular method of list-assisted sampling. The two key determinant s of coverage bias are the proportion of households that are not eligi ble for the sample and the differences in the characteristics of the c overed and not covered populations. The results show that about 4 perc ent of all households are excluded in national samples using this meth od of sampling. Furthermore, they show that the differences between th e covered and uncovered populations are generally not large. The cover age bias resulting from these conditions may often be small.