Agilent Parametric Measurement Basics 2-Parametric Measurement Basic c20130117 [1] free download

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Excerpt Edition This PDF is an excerpt from Chapter 2 of the Parametric Measurement Handbook. The Parametric Measurement Handbook Third Edition March 2012 Chapter 2: Parametric Measurement Basics "I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it ..." "But when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be." -- William Thomson (Lord Kelvin) Measurement terminology Before you can begin to collect measurement data you need to know basic measurement terminology. In particular, understanding the precise meaning of the terms accuracy, repeatability and resolution is essential to understanding parametric test and the measurement capabilities of a parametric measurement resource. Accuracy and repeatability Accuracy and repeatability are related but different quantities. Accuracy is the degree of conformity of a measured or calculated quantity to its actual (true) value. Repeatability (also known as precision) is the degree to which repeated measurements or calculations show the same or similar results. Accuracy and repeatability do not have any innate correlation. A measurement can have high accuracy and high repeatability, high accuracy and low repeatability, low accu- racy and high repeatability, or both low accuracy and low repeatability. The visual analogy that is often used to explain these terms is a bull's-eye target. A group of points that are close to the center of a target but spaced far apart from one another (as shown below) have high accuracy but low repeatability. High accuracy, low repeatability Figure 2.1. High accuracy and low repeatability. 14 A group of points that are far from the center of a target but spaced close together (as shown below) have low accuracy but high repeatability. Low accuracy, high repeatability Figure 2.2. Low accuracy