The U.S. Bureau of the Census and the University of Michigan entered into a Joint Statistical Agreement in the Fall of 1990 to conduct joint research into issues of attrition and time-in-panel bias in the Survey of Income and Program Participation. The JSA was an elaboration of earlier work that examined time-in-panel effects using the 1984 and 1985 SIPP Full Panel Research Files. That prior investigation (JSA 88-16) was directed primarily at other issues, but some investigation of panel bias was conducted on six data items. It was limited by a lack of comparability of data elements between the 1984 and 1985 Full Panel Research files. Despite these limitations, this prior work found some evidence of panel bias, but not uniformly across items. Since only a limited number of items were investigated, and since there were comparability issues raised, it was decided to undertake a more extensive investigation of panel bias in the SIPP.