Retrospective data generally refers to data that has already been collected for another purpose and is being used retrospectively to answer a new research question. This data can come from various sources, such as electronic health records, claims databases, or patient registries, and is often used to generate RWE.

On the other hand, secondary use of existing data refers to the practice of using existing data for a purpose other than the one for which it was originally collected. This can include using data from clinical trials for post-market surveillance or using data from a patient registry for comparative effectiveness research.

While retrospective data can be one type of existing data that is used for secondary purposes, not all secondary uses of data involve retrospective data. For example, prospective data collected for one purpose, such as routine clinical care, can be used for secondary purposes, such as generating RWE.

In summary, retrospective data and secondary use of existing data are related but not interchangeable terms in the context of RWE. Retrospective data is a type of existing data that can be used for secondary purposes, but not all secondary uses of data involve retrospective data.