Panel surveys involve repeated observations made on the same sample of populalation units. In some of these studies systematic biases have appeared; specifically, ifRiis an estimate made from panels appearing in the sample for theithtime, then it has been observed that these estimates sometimes vary systematically withi.It is tempting to interpret these phenomena as being due to conditioning of the panel of respondents. In this article it is shown that these systematic changes can also be a result of the characteristics of the response probabilities. This is true whether the estimates are computed only for those individuals who are identical from one interview time to the next, or on all available persons, or on those persons appearing only once.