Read this and answer the question. If the question is unanswerable, say "unanswerable".

Yale has a history of difficult and prolonged labor negotiations, often culminating in strikes. There have been at least eight strikes since 1968, and The New York Times wrote that Yale has a reputation as having the worst record of labor tension of any university in the U.S. Yale's unusually large endowment exacerbates the tension over wages. Moreover, Yale has been accused of failing to treat workers with respect. In a 2003 strike, however, the university claimed that more union employees were working than striking. Professor David Graeber was 'retired' after he came to the defense of a student who was involved in campus labor issues.

How many strikes has Yale had since 1986?
unanswerable