[工学]SPC Workbook v30.ppt

This page intentionally left blank. INTRODUCTION Process behavior is assessed and adjustments are made based on observed values. The variation in observed values is a function of two components: the variation of the true process and the variation of the measurement process. The variation added by the measurement process can be viewed as measurement error which can be positive or negative. Proper evaluation of observed values can only be accomplished if the measurement process is consistent and judged appropriate for the process being measured. ??? 1 Measurement Systems Analysis INTRODUCTION “In any program of control we must start with observed data; yet data may be either good, bad, or indifferent. Of what value is the theory of control if the observed data going into that theory are bad? That is the question raised again and again by the practical man.” W. A. Shewhart 2 Measurement Systems Analysis Process Measurement Process Physical Outputs Inputs Feedback Observed values = true value + measurement error 3 Measurement Systems Analysis DEFINITIONS Accuracy The difference between a test result and the accepted true value. Bias The difference between the observed average of test results and the accepted reference value. Repeatability The variability of test results obtained with a single measurement instrument when used several times by a single analyst to measure the same sample. Reproducibility The variability in the average of the test results obtained by different analysts using the same measuring instrument on the same sample. 4 Measurement Systems Analysis DEFINITIONS Precision The degree of agreement among individual test results on a single sample. Precision is a combination of repeatability and reproducibility error. Stability The variability of the test results obtained with a measurement system on the same sample over an extended period of time. Tolerance The difference between the upper and lower specification li