This paper describes the relationship between a bathtub curve, failure distributions, and commonly used, but often misunderstood and misapplied, metrics such as the following: mean-time-before failure (MTBF), mean-time-between failure (MTBF), mean-time-to failure (MTTF), and other related terms. A bathtub curve for an object is related to a failure-distribution curve for that object, and when combined, a continuous curve is created. That curve has commonly named regions: infant-mortality, constant-failure, and wear-out, which comprises two subregions that I call degraded-operation and functional-failure. Associated with those curves are numerous metrics that are sometimes misunderstood and misused such as the following: failure rate, prognostic trigger point, prognostic distance (PD), MTBF (two meanings), MTTF, failures-in-time (FIT), and life time (reliability).
