The UC Davis Campus Travel Survey (CTS) is administered annually to a sample of students, faculty, and staff. Because the authors survey only a sample of the campus population and because some groups are more likely to respond to the survey than other groups, it is necessary to apply “expansion factors” and “weights” to the sample to achieve an accurate estimate of the responses for the entire campus population. In effect, the authors use the expansion factors and weights to make the sample of around 4,000 respondents look like the population of around 45,000. The calculation of the expansion factors and weights requires an estimate of the campus population by role group and gender, as explained in more detail below. The campus population is a difficult number to pin down, as it varies over the year and depends on whether and how different categories of people are counted. For the 2016-17 Campus Travel Survey, a new protocol was used to estimate the campus population, as explained in the posted report In reviewing the report, campus officials noticed that the new population protocol produced an underestimate of students living on campus, which significantly changed the estimated mode split and other results. A third protocol was devised to correct the problem, and the authors re-analyzed results from the 2015-16 and 2016-17 surveys using population estimates based on this new protocol.This addendum explains the procedure for expansion factors and weights, describes the new population estimation protocol, and presents the revised results for selected tables from the CTS reports.
