Need assurance that a population of 185 devices are OPERABLE. A MODEL has been developed for prediction of each devices condition. Periodically, it is necessary to TEST the devices in case the MODEL is wrong. However, it is expensive to TEST these devices. Therefore, it would make sense to periodically TEST only a subset of the population. How should I select a subset to ensure reasonable confidence (95%) that all devices are OPERABLE? My thoughts are that I could randomly select 20 of the devices to TEST. I could then calculate the STDEV and CORREL between the MODEL and TEST results. If the STDEV is sufficiently small enough and the CORREL is almost 1.000, then I should be able to use the MODEL to determine which devices to TEST. However, I'm not sure how to establish acceptable values. Alternatively, how could the MODEL be modified or biased so that it could be used to determine the population for periodic testing? Many thanks for all responses!