Evaluating the effect of accuracy ratios on the percent of calibrations which are out of tolerance

The standard practice in calibration laboratories across the country, including the Measurement Standards and Calibration Laboratory (MSCL) at the Johnson Space Center, is to use accuracy ratios to determine if instruments are in-tolerance rather than computing the actual uncertainty associated with the instruments. In the past, the accepted practice was to use an accuracy ratio of 10:1, but then state of the art advanced to the point where the 10:1 ratio could no longer be maintained, and the ratio was arbitrarily lowered to 4:1. It is now becoming increasingly difficult to maintain the 4:1 accuracy ratio, and in some cases 1:1 is the best that can be achieved. However, the effect of using these small accuracy ratios on the number of mistakes made in classifying instruments as in or out of tolerance is completely unknown. In order to assess the effect of using accuracy ratios in calibration, a simulation program was written to compute the proportion of instruments determined to be out of tolerance which were actually in, denoted by alpha, and the proportion of instruments determined to be in-tolerance which were actually out, denoted by beta. This was done for accuracy ratios of 1:1 and 10:1, for one to five progressive calibrations, under varying standard and instrument conditions. Selected results are presented and explained.