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Accuracy vs. Precision in Chemistry: Definitions & Comparisons

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  • 0:05 Definition of Accuracy…
  • 1:19 Comparing Accuracy and…
  • 3:09 Lesson Summary
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Lesson Transcript
Instructor: Sara McCubbins

Sara has a background in chemistry education and is currently writing her dissertation in the field of curriculum and instruction.

Accuracy and precision are words that are often used interchangeably, but in science they have very specific and different definitions. In this lesson, we will explore the ways in which these terms are used in chemistry as well as explore comparisons and differences.

Definition of Accuracy and Precision

Charlotte loves to play basketball. She has been practicing her free throws in order to try and make the basketball team. Before she started practicing, she was hitting the top right corner of the backboard for eight out of ten shots. Her shots were precise, but they were not making the basket, so they were not accurate. After practicing, she made seven out of ten free throws. Now, her shots were both precise and accurate! In chemistry, the slight difference between these two terms is very important.

Charlotte's shots were accurate when they went in the basket and the end result was equal or close to the expected outcome (making the basket). The same is true in chemistry when we talk about measurements. In chemistry, accuracy refers to how close a measurement is to its standard or known value.

When Charlotte was hitting the top right corner of the backboard repeatedly, her shots were precise because they were occurring in the same area, but they were not accurate because she was not making the basket. In chemistry, the same is true when we talk about precision of measurements. Precision refers to how close two or more measurements are to each other, regardless of whether those measurements are accurate or not. It is possible for measurements to be precise but not accurate.

Comparing Accuracy and Precision in Chemistry

When we think of accuracy and precision in chemistry, sometimes it helps to imagine a bullseye like on the targets shown here. In (a) we can see that the dots are spread across the bullseye, so they are neither accurate nor precise. In (b) we can see that all the dots are centered around the bullseye, so they are both accurate and precise. In (c) we can see that the dots are close together at the bottom of the bullseye, so they are precise but not accurate.

(a) not accurate, not precise, (b) both accurate and precise, (c) precise, not accurate
accuracy vs precision

Because accuracy and precision are dealing with measurements, we see these terms most often in reference to the laboratory setting. Imagine a chemist who is conducting an experiment to see how many grams of Substance X she can produce from a given chemical reaction. She knows, based on established studies, that she should yield 7.4 grams of Substance X. She conducts the experiment three times and receives a yield of 5.2 grams, 4.9 grams, and 5.1 grams. Because these results are all close to each other, we would consider them to be precise. However, they are not close to the expected yield of 7.4 grams, so we would say that the results are not accurate.

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