Many times longitudinal surveys have to answer a question on how to combine differing cohorts of the survey. These different cohorts can be things such as disjoint populations, a single population or overlapping populations. Former NISS postdoc Ivan A. Carrillo and NISS Director, Alan F. Karr, wrote a paper suggesting a course of action for combining different cohorts in a longitudinal survey with a repeated panel design. The paper looks at the Survey of Doctorate Recipients, conducted by the U.S. National Science Foundation. The technical report can be found here.