Comparisons of exponential life test plans with intermittent inspections

Abstract

Most of the existing life test plans are concerned with life tests conducted under continuous inspection of test items. However, a reduction in testing effort and administrative convenience may be achieved by employing intermittent inspection in which items are inspected only at certain points in time. The well-known hybrid plan, which employs a single inspection at the censoring time, is frequently used in practice. To investigate whether or not adding more inspections to the hybrid plan improves its statistical performance substantially, a life test plan with multiple inspections is first developed assuming that the lifetimes of items follow an exponential distribution. For multiple inspection times, equally-spaced, equal-probability, and equally-spaced in log-time inspection schemes are considered. The developed plan is then compared to the hybrid and the Type I-censored life test plan with continuous inspection in terms of the sample size required, the expected completion time to reach a decision, and the power of test. Computational results indicate that the above performance measures are relatively insensitive to the inspection scheme employed, which implies that the equally-spaced inspection scheme may be considered as an appropriate choice since it is more convenient to schedule and does not require a priori information on unknown parameters. Another important finding is that the life test plan with multiple inspections is preferred when the discrimination ratio is small and the censoring time is large. Otherwise, conducting more than one inspection does not improve statistical performances substantially, and the hybrid plan is generally recommended.