Advantages and disadvantages : longitudinal vs. repeated cross-section surveys

The collection of panel data entails following a cohort of individuals with the purpose of monitoring changes over a period of time. For the Puget Sound Transportation Panel (PSTP) data, households were solicited to provide a two-day travel diary for each survey time period. The survey time period is referred to as a “wave,” and a total of five waves of data have now been collected at approximately one year intervals. Ideally, in any longitudinal study, the same group of subjects (individuals) is followed during each wave, thus making possible the observation of any one individual’s travel behavior over time. Longitudinal surveys differ greatly from the collection of repeated cross-sectional data in which an independent sample is collected at each wave to represent the population for that time period. With cross-sectional data, the observed trip information is representative of the population at a single period in time and the temporal aspects of a specific individual’s travel is not necessarily available. In the following discussion we have focused on several pertinent issues regarding the scope and limits of statistical inferences for the two types of data that result from longitudinal and crosssection surveys.