Alexandra Wood et al. have self-archived "Integrating Approaches to Privacy Across the Research Lifecycle: Long-Term Longitudinal Studies."
Here's an excerpt:
The defining characteristics of a long-term longitudinal study—that such a study follows a large number of individuals over extended periods of time and collects vast quantities of sometimes very sensitive personal information from subjects—create a number of challenges for researchers working with longitudinal data. Over the course of a longitudinal study, data collection, uses of data, privacy expectations, and laws may change, leaving the scope of subjects' consent unclear. The use of contractors for data collection and the need to retain administrative data linked to records to recontact subjects over time may increase the risk of disclosure.