The mean time between failures (MTBF) of SATA drives is usually specified to be about 1.2 million hours (some drives such as Western Digital Raptor have rated 1.4 million hours MTBF), while SAS/FC drives are rated for upwards of 1.6 million hours. However, independent research indicates that MTBF is not a reliable estimate of a drive's longevity (service life). MTBF is conducted in laboratory environments in test chambers and is an important metric to determine the quality of a disk drive, but is designed to only measure the relatively constant failure rate over the service life of the drive (the middle of the "bathtub curve") before final wear-out phase. A more interpretable, but equivalent, metric to MTBF is annualized failure rate (AFR). AFR is the percentage of drive failures expected per year. Both AFR and MTBF tend to measure reliability only in the initial part of the life of a hard disk drive thereby understating the real probability of failure of a used drive.