Key concept · Evidence

Unit 3: Measurement and Scientific Evidence

Physics is built on evidence from careful measurement. This unit teaches you how to plan a fair investigation, take accurate measurements, and judge how trustworthy your data is.

01 Types of variable

Variable	Meaning
Independent	The one variable you deliberately change
Dependent	The variable you measure, which changes because of the independent variable
Controlled	Variables you keep the same, so the test is fair

Fair test: only the independent variable should change between trials. Every controlled variable must stay the same.

02 Accuracy vs. precision

Accuracy means how close a measurement is to the true, correct value.

Precision means how close repeated measurements are to each other (even if they are not close to the true value).

Think of shooting arrows at a target: arrows grouped tightly in the bullseye = accurate and precise. Arrows grouped tightly but away from the bullseye = precise but not accurate. Arrows scattered but centred on the bullseye on average = accurate but not precise.

03 Uncertainty and repeats

No measurement is perfect. There is always some uncertainty from the limits of your instrument and from human error. To reduce the effect of random error:

Repeat each measurement (usually 3 times) and calculate the mean (average)
Identify and remove clear outliers (anomalous results) before averaging
Use instruments with finer scales when possible

04 Presenting evidence

Good evidence is organised clearly: a labelled table with correct units, and a graph with labelled axes, units, a sensible scale, and a line or curve of best fit (not just dot-to-dot).

V Key vocabulary

Term	Meaning
Independent variable	The variable you choose to change in an experiment.
Dependent variable	The variable you measure as a result.
Anomaly	A result that does not fit the general pattern of the data.
Mean	The average of a set of repeated results.

? Quick check

Try to answer before you tap. Answers are hidden underneath.

A student tests how the length of a pendulum affects its time period. Which is the independent variable?

The length of the pendulum — it is the variable the student deliberately changes.

Why should you repeat measurements and take a mean?

Repeating reduces the effect of random error and one-off mistakes, making the result more reliable.

What is the difference between accuracy and precision?

Accuracy is how close a result is to the true value. Precision is how close repeated results are to each other, whether or not they are close to the true value.

← previousUnit 02: Forces and How They Interact next →Unit 04: Describing Motion (Kinematics)