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What do the terms 'accuracy' and 'precision' mean in chemistry? Learn the definition and significance of accuracy and precision. See differences between them. Updated: 08/24/2021
Accuracy Definition
In chemistry, accuracy is defined as how close a measurement results comes to the true value. In other words, accuracy is the ability of the instrument to measure the accurate value or the closeness of the measured value to a standard or true value.
Types of Accuracy
Accuracy has been classified into 3 categories:
- Point Accuracy
- Accuracy as Percentage of True Value
- Accuracy as Percentage of Scale Range
High Accuracy, Low Precision Example
Point Accuracy
Point accuracy means the accuracy of the instrument is only at the particular point on its scale. However, this type of accuracy does not give any information about the general accuracy of the instrument.
Mass measurement is a good example of point accuracy. Mass can be measured on a Mettler scale with accuracy within a fraction of a gram depending on how well the scale is calibrated. If a home scale is used to measure mass, it usually needs to be tared (zeroed) to calibrate it. For a scale used to measure weight, the value could be off by half a pound or more, plus the accuracy of the scale may change depending on where the object is in the instrument's range. A person weighing close to 125 pounds might get a more accurate measurement than a baby weighing 12 pounds.
Accuracy: Percentage of True Value
Percentage of true value is when the accuracy of the instrument is determined by identifying the measured value regarding their true value. The accuracy of the instruments is neglected up to ±0.5 percent from the true value.
For example, a cube is measured that is known to be 10.0 inches cross and its values are 9.0 inches, 8.8 inches, and 11.2 inches, these values are more accurate than if the values were 11.5 inches, 11.6 inches, and 11.6 inches (which are more precise).
Accuracy: Percentage of Scale Range
Percentage of scale range determines the accuracy of a measurement. Here is an example that might help with this definition.
A thermometer has a scale range up to {eq}500^{o}C {/eq}. The thermometer has an accuracy of {eq}\pm 0.5 {/eq}, i.e. {eq}\pm 0.5 {/eq} percent of increase or decrease in the value of the instrument is negligible. But if the reading is more or less than {eq}0.5^{0}C {/eq}, it is considered a high-value error.
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.
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Precision Definition
In chemistry, precision refers to the reproducibility of a result or measurement. In other words, precision refers to the closeness of two or more measurements to each other.
An example would be if a given substance was weighed five times and the result was 3.2 kg each time, then the measurement is very precise but not necessarily accurate. Precision is independent of accuracy.
Another example is if the measurement of a mass of a body is 20 kg and the measurement is 17.4, 17, 17.3, and 17.1, the weighing scale is precise, but not very accurate. If the scale gives values of 19.8, 20.5, 21.0, and 19.6, it is more accurate than the first balance, but not very precise.
Measurements can be both accurate and precise, accurate but not precise, precise but not accurate, or neither.
High Precision, High Accuracy Example
Two different elements of precision include:
- Repeatability
- Reproducibility
Repeatability
Repeatability is the variation arising within conditions (location, instrument, observer) that are kept identical when repeated measurements are taken during a brief time period. To calculate repeatability, conduct the same experiment multiple times and perform a statistical analysis on the results.
Expanding on a previously used example, a given substance is weighed five times using the same instrument over a 10-minute timeframe and gets 3.2 kg each time. This is repeatability.
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 accurateDefinition 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- Activities
- FAQs
Accuracy and Precision
Science involves gathering data and interpreting it. Sometimes data collected shows a trend and other times it doesn't show a trend. Knowing how to interpret data is key in chemistry and every other science. Let's do an activity demonstrating the difference between accuracy and precision.
Materials Required
- two pieces of white paper
- two pieces of any other color paper (white is okay too)
- scissors
- a compass to draw circles or some round objects of varying sizes to trace
- a pencil
- a marker
Procedure
- Draw identical targets on both pieces of paper.
- Cut out a circle on the colored piece of paper that is the same size as the bulls eye on the white piece of paper.
- Cut out a small area (roughly half the size of your hand) of the other piece of colored paper in a random location and make it whatever shape you want.
- Put the colored piece of paper with the bulls eye cut out on the white target paper.
- Pick up the marker, close your eyes and start making marks on the colored piece of paper. Use your imagination to try to get as many bulls eyes as you can.
- Do the same thing with the other piece of colored paper on the other target.
- Remove the colored pieces of paper from the targets.
Analysis
- How many bulls eyes did you make on the target that only had the bulls eye exposed?
- How many bulls eyes did you make on the target that had the random shape and location of the target exposed?
- How does the bulls eye exposed target demonstrate accuracy?
- How does the other target with the random shaped exposure demonstrate precision?
Solutions
The answers to questions 3 and 4 are as follows:
3. Since the bulls eye was the only exposed surface of the target every strike with the marker was either a bulls eye or not. Each bulls eye strike is labeled accurate.
4. Since all of the marks on the random shape exposure target were clustered in this area it demonstrates precision. Precision are closely grouped data points.
What is precision in chemistry?
Precision refers to the closeness of multiple measurements to each other. In other words, it expresses the degree of reproducibility or agreement between repeated measurements. .
What is difference between precision and accuracy?
Accuracy is how close a value is to its true value. An example is how close an arrow gets to a bull's-eye center. Precision is how repeatable a measurement is. An example is how close a second arrow is to the first one (regardless of whether either is near the mark).
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