Scientific English as a Foreign Language
Answers to Lesson of June 14, 1999
Accuracy and Precision

In some languages, there is no distinction between accuracy and precision. But in scientific English they mean different things. Accuracy is the degree to which a result agrees with the theoretical value. Precision indicates how well that result can be repeated. If you think of archery, an accurate imprecise archer will shoot a series of arrows; the average of the series of shots will be the center of the target, but the individual arrows are scattered over the target's surface. A precise inaccurate archer will be able to repeat the position of the arrows very well, but the position of the arrows averaged over a number of shots will be far from the center of the target. A precise accurate archer hits the center of the target every time. Both accuracy and precision are useful to know when evaluating experimental results, especially when introducing a new technique or measuring fundamental constants.

Do the sentences below indicate accuracy, precision, or both?

1. As the gun warmed up, it was hard to repeatedly hit the target. precision
2. The students reported that the speed of light is 2 x 10^8 m/s, an error of -33%. accuracy
3. The rest mass of the neutron is 1.008 665 012 amu (atomic mass units) with 0.037 ppm (parts per million) error. accuracy and precision
4. Our mean value was 50 GPa; our standard deviation was 5 GPa. precision
5. The R value from the least-squares fit was 0.987. precision
6. We used a standard to calibrate our instrument. We think that the values we obtained are good to within 5%. accuracy
7. If you can measure the fine structure constant to three significant figures (137), then you don't have a very good instrument. However, if you can measure it to nine significant figures (137.035 963), then I want to see your lab! precision
8. The reproducibility of the experiment is a few degrees Centigrade. The temperature can be measured to plus or minus five degrees. accuracy and precision

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Created June 14, 1999, by Nancy Burnham and Fred Hutson.