Theme - a
comparison test which involves a check of the hypothesis that the Mean Time
Between Failures of a "new" item (MTBFn) - or the mean
time to failure (MTTFn) - is not less that of a counterpart
taken as basis for comparison (MTBFb or MTTFb
). It is assumed that the MTBF is distributed exponentially and that
information is available on a known quantity (rb) of TBF's
for the basic item. The Sequential Probability Ratio Test (SPRT) is widely used
for checking that an MTBF is not less than a specified value. Here, the
innovation is that allowance is made for the uncertainty in the MTBFb
estimate, which increases as rb decreases. Accordingly, a
search algorithm was developed for the decision boundaries in the proposed
test, as well as for calculating the Operating Characteristic (OC), and the
statistical parameters of the test duration and number of failures to stopping
the test. Approximative relationships were obtained for the above, depending on
the target OC. These straightforward relationships not only simplify the
planning process, but will convince the reliability engineer of the
indispensability of allowing for the above uncertainty, and of the effectiveness
of the proposed test.
