Accurate data on the number of students who transfer from two-year colleges to senior colleges are not available because the ways of counting transfers vary greatly from system to system and state to state. While most studies define transfer students as those who have taken courses at a community college and subsequently enroll at a senior institution, there is little agreement as to the number of units that must be taken before the student achieves transfer status. Statewide definitions range from 30 semester credit hours to one quarter hour of credit--some states set no credit hour limits at all. Methods of collecting the data on transfers also vary depending upon whether the researcher is based at a community college or at a state office. Data obtained by college-based researchers are frequently invalidated by low response rates to surveys due to the high mobility of community college students. Nonreturning students and interstate transfers are especially difficult to trace. In cases where the community colleges rely exclusively upon the universities in their region for transfer information, figures are particularly misleading due to missing and incomplete data and other imbalances caused by variations in data collection methods. Tracing students who "stop in" and "stop out" of different types of postsecondary institutions continues to be a problem as data collection systems do not accommodate them. A bibliography is included. (DR)