Slide 1 "If you have only one watch, you always know exactly what time it is. If you have two watches, you are never quite sure..." Slide 2 When we take a single reading we tend to accept the readout without really thinking about its validity. People seem to do this even when they know the reading is inaccurate. Your digital watch is probably off by a minute or two right now! Slide 3 Every measurement is subject to errors and inaccuracies. Modern digital instruments convey such an aura of accuracy and reliability, that we forget this basic rule... There is no such thing as a Perfect Measurement Slide 4 Talking about Measurement The result obtained from counting will be an exact result (barring blunder) The result obtained from measuring will only approach the truth Slide 5 Talking about Measurement Precision Accuracy Resolution Slide 6 Talking about Measurement “Accuracy is telling the truth . . . Precision is telling the same story over and over again.” -Yiding Wang Slide 7 Accuracy A measurement is accurate if it correctly reflects the size of the thing being measured. Accurate means "capable of providing a correct reading or measurement." In physical science it means 'correct'. The degree of conformity with a standard. Accuracy relates to the quality of a result. Slide 8 Precision Precision refers to the reproducibility of a result or measurement. In physical science it means "repeatable, reliable, getting the same measurement each time." The degree of refinement in the performance of an operation, or the degree of perfection in the instruments and methods used to obtain a result. Slide 9 Talking about Measurement To understand the difference between these two properties, let us use the analogy of the marksman who uses a gun to fire bullets at a target. In this analogy, the gun is the instrument, the marksman is the operator of the instrument, and the results are determined by the location of the bullet holes in the target. Slide 10 In this figure, we see that the marksman's instrument was quite precise, since his results were uniform. The instrument did not provide accurate results since the shots were not centered on the target's bull's eye. The fact that his results were precise, but not accurate, could be due to a consistent operator error. Therefore precision tells us something about the quality of the instrument's operation. Precision Slide 11 In this figure, we see the shots are centered on the bull's eye but the results were not uniform. This indicates that the marksman's instrument displayed good accuracy but poor precision. In this case, the marksman will never achieve both accuracy and precision. If he is not satisfied with the results he must change his equipment. Therefore accuracy tells us something about the quality or correctness of the result. Accuracy Slide 12 Accuracy & Precision As scientists, we desire our results to be both precise and accurate. As shown in this figure, the shots are all uniform and centered on the bull's eye. This differs from the first figure in that the marksman has compensated for the error. Slide 13 Accuracy & Precision Slide 14 Slide 15 Slide 16 Slide 17 Slide 18 Resolution Resolution is the ability of the measurement system to detect and faithfully indicate small changes in the characteristic of the measurement result Slide 19 Resolution The smallest interval that can be measured. Tools with smaller resolution will generally have greater accuracy.