What Is the Difference Between Accuracy and Precision?

Accuracy and Precision
Accuracy is how close a measurement is to the true value while precision is close measurements are to each other.

Accuracy and precision are two important concepts in science. Both terms apply to any experimental measurement you make. But what do they actually mean? What is the difference between them and which is more important?

What Is Accuracy?

Shots with high accuracy, low precision

Accuracy is a measure of how closely your experimental measurements agree with known values. The closer your measurements are to the known value, the more accurate the measurement.

If you think of the known value as the bullseye on a target, accurate shots (or measurements) mean the average of your shots is at or near the bullseye. The shots in this picture are accurate, but they are all over the target. This brings up the concept of precision.

What Is Precision?

Shots with high precision, low accuracy.
Shots with high precision, low accuracy.

Precision is a measure of how close your experimental measurements agree with each other. The closer each measurement is to the other measurements, the more precise your measurement.

If the measurements were shots in a target, the closer the holes are to each other, the more precise the shots. Here, the shots are all clustered together. They would be considered a precise collection of shots. The only problem is they are not very accurate.

How to Control Accuracy and Precision

Shots with high accuracy and precision.
Shots with high accuracy and precision are the goal.

Both accuracy and precision are the goal of any measurement. Variations in accuracy and precision are largely controllable.

In target shooting, you improve accuracy by moving closer to the target, or using an aiming aid like a scope or laser pointer. You improve precision by mounting your gun to a table or bench or shooting indoors out of the wind.

In a lab, improve accuracy and precision by using the same procedure taking each measurement and using tools designed for measurements. For example, you get a more accurate and precise volume measurement using a graduated cylinder than you do using a water glass.

In general, use good tools designed for the measurement you are trying to make and pay close attention to what you are doing when you use them. This helps any measurement you make.

Which Is More Important? Accuracy or Precision?

Shots that are neither accurate nor precise.
Shots that are neither accurate nor precise are useless.

Accuracy is more important when trying to hit a target. You either hit your target or you don’t. It does no good to be precise if you miss all your shots. You can improve accuracy in future measurements by factoring in a correction factor. If your shots are all going to the left, alter your aim to the right. Accuracy is something you can fix in future measurements.

Precision is more important in calculations. When using a measured value in a calculation, you can only be as precise as your least precise measurement. This is the main idea behind the topic of significant figures in calculations. You can improve precision by using a better measuring tool and/or improving your skill at using the tool.

Accuracy and precision are both important to good measurements in science. Try to make the best of both in all your measurements. But, no matter how hard you try, measurements always deviate slightly from true values. This is because of error.