Chapter 5
The general kinds of uncertainty in any measurement
5.1 Random or systematic
5.2 Distribution - the ‘shape’ of the errors
5.3 What is not a measurement uncertainty
5.3 What is not a measurement uncertainty
Mistakes made by operators are not measurement uncertainties. They should not be counted as contributing to uncertainty. They should be avoided by working carefully and by checking work.
Tolerances are not uncertainties. They are acceptance limits which are chosen for a process or a product. (See Section 10 below, about compliance with specifications).
Specifications are not uncertainties. A specification tells you what you can expect from a product. It may be very wide-ranging, including ‘non-technical’ qualities of the item, such as its appearance. (See Section 10 below).
Accuracy (or rather inaccuracy) is not the same as uncertainty. Unfortunately, usage of these words is often confused. Correctly speaking, ‘accuracy’ is a qualitative term (e.g., you could say that a measurement was ‘accurate’ or ‘not accurate’). Uncertainty is quantitative. When a ‘plus or minus’ figure is quoted, it may be called an uncertainty, but not an accuracy.
Errors are not the same as uncertainties (even though it has been common in the past to use the words interchangeably in phrases like ‘error analyses). See the earlier comments in Section 2.3.
Statistical analysis is not the same as uncertainty analysis. Statistics can be used to draw all kinds of conclusions which do not by themselves tell us anything about uncertainty. Uncertainty analysis is only one of the uses of statistics.