22 min read

Accuracy vs Precision

Accuracy is closeness to the true value. Precision is how tightly repeated results cluster — they are not the same idea.

G1 — What Accuracy and Precision Mean

Two words that sound similar, but ask different questions about a set of results: Accuracy = how close a result (or the average of several results) is to the true / target value. Precision = how close repeated results are to each other, whether or not they are near the true value. Key idea: accuracy is about being right; precision is about being consistent . You can have one without the other.

Think of each measurement as a shot on a target. The bullseye is the true value . Looking at a cluster of shots, you can name both accuracy and precision.

Under the four panels: 1 High accuracy + high precision — tight cluster right on the bullseye. 2 High accuracy + low precision — shots scattered, but centred roughly on the bullseye on average. 3 Low accuracy + high precision — tight cluster off to one side. 4 Low accuracy + low precision — scattered widely and off-centre.

Example 1 — Worked example — reading panel 3

Look at Low accuracy, high precision (bottom-left panel): The shots are close together \to high precision . The whole cluster sits away from the centre \to low accuracy . So this panel is high precision, low accuracy — consistent, but systematically wrong (like a scale that is always 0.5 kg too heavy).

Practice. Classify each practice target below as one of the four combinations.

Practice A — answer: tight cluster away from the bullseye \to low accuracy, high precision . Practice B — answer: shots scattered but roughly around the centre \to high accuracy, low precision .

Practice

Repeated lab readings of the same mass are 2.01 kg, 2.00 kg, 2.01 kg, but the true mass is 1.50 kg. Which description fits?

G1 — What Accuracy and Precision Mean

Two words that sound similar, but ask different questions about a set of results: Accuracy = how close a result (or the average of several results) is to the true / target value. Precision = how close repeated results are to each other, whether or not they are near the true value. Key idea: accuracy is about being right; precision is about being consistent . You can have one without the other.

G2 — The Four Combinations on a Target

Think of each measurement as a shot on a target. The bullseye is the true value . Looking at a cluster of shots, you can name both accuracy and precision.

Under the four panels: 1 High accuracy + high precision — tight cluster right on the bullseye. 2 High accuracy + low precision — shots scattered, but centred roughly on the bullseye on average. 3 Low accuracy + high precision — tight cluster off to one side. 4 Low accuracy + low precision — scattered widely and off-centre.

Example 1 — Worked example — reading panel 3

Look at Low accuracy, high precision (bottom-left panel): The shots are close together \to high precision . The whole cluster sits away from the centre \to low accuracy . So this panel is high precision, low accuracy — consistent, but systematically wrong (like a scale that is always 0.5 kg too heavy).

Practice. Classify each practice target below as one of the four combinations.

Practice A — answer: tight cluster away from the bullseye \to low accuracy, high precision . Practice B — answer: shots scattered but roughly around the centre \to high accuracy, low precision .

Practice

Repeated lab readings of the same mass are 2.01 kg, 2.00 kg, 2.01 kg, but the true mass is 1.50 kg. Which description fits?