Annualized failure rate (AFR) gives the estimated probability that a device or component will fail during a full year of use. It is a relation between the mean time between failure (MTBF) and the hours that a number of devices are run per year. AFR is estimated from a sample of like components—AFR and MTBF as given by vendors are population statistics that can not predict the behaviour of an individual unit.
Hard disk drives
For example, AFR is used to characterize the reliability of hard disk drives.
The relationship between AFR and MTBF is:
This equation assumes that the drives are powered on for the full 8760 hours of a year, and gives the estimated fraction of an original sample of drives that will suffer from disk failures, or, equivalently, 1 − AFR is the fraction of drives that will show no failures over a year. It is based on an exponential failure distribution (see Failure rate for a full derivation).
This can be approximated by, assuming a small AFR,
For example, a common specification for PATA and SATA drives may be 300,000 MTBF, giving a theoretical 2.92% annualized failure rate i.e. a 2.92% chance that a given drive will fail during a year of use.
The AFR for a drive is derived from time-to-fail data from a reliability-demonstration test (RDT).
AFR will increase towards and beyond the end of the service life of a device or component. Google's 2007 study found, based on a large field sample of drives, that actual AFRs for individual drives ranged from 1.7% for first year drives to over 8.6% for three-year-old drives. A CMU 2007 study showed an estimated 3% mean AFR over 1–5 years based on replacement logs for a large sample of drives.