STATISTICAL-ANALYSIS OF DEADBAND-DEVIATION RULES IN QUALITY-CONTROL

The paper is concerned with the statistical analysis of rules designed to handle deviations from a prescribed tolerance interval, named a de adband, around a desired specification for a production process. The r ules are ostensibly designated to recalibrate the process at the inten ded specification and as such reflect a certain interpretation of the deviation from it-either a machine shift or a random error or any comb ination thereof. The true nature of the deviations, however, is always unknown. Statistically we assume that any actual measurement is symme trically distributed around the last calibrated value of the productio n process and that the repeated calibrations only shift around the loc ation of the said distribution but do not alter its shape. The analysi s is done under the hypothesis that all deviations are indeed random e rrors and are not due to any machine shift, namely, a null hypothesis type background. We investigate the principal parameters engendered by the calibration rules. First we prove that the repeated, and possibly frequent, adjustments to the production process do not Fundamentally affect it in the sense that the expectation of any calibrated value re mains the original specification of the process. This property is cruc ial since otherwise the calibration rules would have generated bias re ndering them useless. We next develop an expression for the variance o f the calibrated values. Because of the prohibitive form of the latter we resort to simulation in order to explore its behavior. The paper e nds with a suggested plan for the study of the same parameters under a lternative type hypotheses.