Correction of Data for Errors of Measurement

V E R Y measurement is subject to error. This universally accepted truth is the result of every-day experience. From the simplest type of measurement, such as determining the length of a board with an ordinary tape measure, to the most refined type of measurement, such as determining the charge on an electron, errors are bound to creep in. Now, a manufacturer must constantly make measurements of one kind or another in an effort to control his production processes and to measure the quality of his finished product in terms of certain of its characteristics, but, before he can safely determine the significance of observed differences in his production processes or in the quality of his product as given by these measurements, he must make allowance for his errors of measurement; i.e., for the fact that the observed differences may be larger or smaller than the true differences. To make such allowances for the errors of measurement of any characteristic, t o find o u t w h a t the t r u e m a g n i t u d e of the characteristic most probably is, to find out, as it were, what a thing most probably is from what it appears to be, presents an endless chain of interesting problems to be solved. Three important types of problems arising in engineering practice are discussed in this paper. They are: 1.' Error correction of data taken to show the quality of a particular lot. 2. Error correction of data taken periodically to detect significant changes in quality of product. 3. Error correction of data taken to relate observed deviations in quality of product to some particular cause.