The purpose of S.M.A.R.T. is to warn a user or system administrator that a drive is about to fail. At the time of the warning, there is usually still time to do certain things to prevent data loss, for example copying the data to a different drive. Approximately 30% of failures can be predicted by S.M.A.R.T. Work at Google on over 100,000 drives has shown little overall predictive value of S.M.A.R.T. status as a whole. The study suggests that certain sub-categories of information which some S.M.A.R.T. implementations track "do" correlate with actual failure rates. In the 60 days after the first scan error on a drive, the drive is 39 times more likely to fail on average than it would have been had no such error occurred. Also, first errors in reallocations, offline reallocations and probational counts are strongly correlated to higher probabilities of failure.