Typical disk drives for personal computers have MTBF ratings of about 500,000 hours. This means that of all the drives tested, one failure occurred every 500,000 hours of testing. Disk drives are typically tested only a few hours, and it would be unlikely for a failure to occur during this short testing period. Because of this, MTBF ratings are also predicted based on product experience or by analyzing known factors such as raw data supplied by the manufacturer.