◆ SpookStack

hardware improvements that have taken place through the years. For samples approaching 100 or so, it seriously over-weights the old data and under-weights the more recent events. Although equal weighting does not seem suitable for this application, it could be appropriate in other large-sample situations, for example, predicting the failure probability of devices that are all manufactured at the same time by the same process, and tested to the same standards. Table 4. Comparison of Weicllting Percentages Sample Size Filter* Last+ Point Last5 Points Last 10 Points Last 25 Points !Last 50 Points Last Half 4 Expon. Index Equal 25.8 40.0 25.0 -- - --- --- --- 51.0 70.0 50.0 10 Expon. Index Equal 10.9 18.2 10.0 52.5 72.7 50.0 100.0 100.0 100.0 --- --- 52.5 72.5 50.0 20 Expon. Index Equal 6.0 9.5 5.0 28.9 42.9 25.0 55.0 73.8 50.0 --- --- 55.0 73.8 50.0 100 Expon. · Index Equal 2.3 2.0 1.0 11.1 9.7 5.0 21.1 18.9 10.0 45.7 43.6 25.0 73.3 74.8 50.0 73.3 74.8 50.0 200 Expon. Index Equal 2.0 1.0 0.5 9.8 4.9 2.5 18.6 9.7 5.0 40.4 23.4 12.5 64.7 43.7 25.0 88.3 74.9 50.0 500 Expon. Index Equal 2.0 0.4 0.2 9.6 2.0 1.0 18.3 4.0 2.0 39.7 9.7 5.0 63.6 19.0 10.0 99.4 75.0 50.0 1000 Expon. Index Equal 2.0 0.1 0.1 9.6 1.0 0.5 18.3 2.0 1.0 39.7 4.9 2.5 63.6 9.7 5.0 99.996 75.0 50.0 * F = 0.98 for exponential filter + "Last" refers to the most recent data point The index-count filter has serious deficiencies when applied to either small or large samples of missiles and space vehicles. For small samples, too much emphasis is placed on recent data. For a sample of four, 40% of the total weight is given to the last test, and 70% to the last two tests. For a sample of ten, 18.2% of the total weight is given to the last test and 72.7% to the last five tests. The reliability improvement rate implied by these weightings seems too optimistic unless there were serious design flaws in the early configurations that were discovered and corrected. Since many types of failures surely exist that occur only once in 50 or once in 100 or more launches, the tenth launch may be no better than the first for predicting the probability of occurrence of such failures. For large samples, the index-count filter under-weights current data 9/10/96 19 RTI