Passive data are data collected without the involvement of respondents. They can supplement or replace survey data to improve data quality. For example, rather than asking respondents how often they attend exerise (physical activity is often overreported in survey), we could give them fitness trackers that record their steps and heart rate.
I’m intersted in what variables we can measure passively, for which respondents, and how good the passive measures are.
In some cases, the best approach may be to combine survey and passive measures, since both are subject to error. My JASA paper discusses this approach.