In our earlier article on measuring change, we discussed longitudinal designs involving two surveys with the same participants. However, longitudinal surveys need not be limited to only two surveys and may continue for many “waves”. For example, the Household Income and Labour Dynamics of Australia (HILDA) Survey has been conducted on an annual basis since 2001 and is still ongoing.
Of course, while longitudinal surveys try to include the same sample every time, there are always some who drop out – sometimes permanently, but sometimes only for one or two waves. As with many one-off or “cross-sectional” surveys, longitudinal survey data often includes weights that can be applied when using the results to make inferences about the population as a whole. The weights effectively scale the sample up to the population. Since the number of respondents will vary from wave to wave, a given respondent is likely to have different weights assigned to their responses for different waves. In addition, there may be multiple sets of weights, with consideration of the analysis being undertaken necessary to determine which set of weights to use.