For most processes, we have a choice of measurement options that vary with cost. Ideally, we seek the most accurate measurement at the lowest cost with the expectation that the result will be satisfactory. When measurements are critical to operations, we should validate these assumptions.
We might measure a quantity that is not critical to our operations but is easy to measure to serve as a proxy for a parameter of interest. For example, a bottle filling station may be concerned with fill volume, but measure weight since it’s easier.