Annualized failure rate
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Annualized failure rate, or short AFR is the reciprocal of the MTBF expressed in years and percent.
For example, a typical disk drive MTBF number may be 1,200,000 hours. Divide that by 8,760 hours to get 136,9863 years, then take the reciprocal of 136,9863 years to get 0.73%. You can expect about 0.73 percent of the population to fail in the average year.