The weakness of pseudonymized data to Inference attacks is commonly overlooked.

Second, given how quickly information search tools evolve, you should assume that your pseudonymized messages will become easier to associate with you over time.

Protecting statistically useful pseudonymized data from re-identification requires:

The book was an account of the working life of a nurse, based on an pseudonymized series of interviews with a 27-year-old nurse named Philadelphia.

For example, given prior knowledge of a few attendance dates it is easy to identify someone's data in a pseudonymized dataset by selecting only those people with that pattern of dates.