Accuracy, precision and errors

No measurement is perfect. Good scientists know how their results might be wrong and how to make them better.

Practise this lesson free (7 questions)

Accurate and precise

Accurate: close to the true value.
Precise: repeated readings are close to each other (little spread).
Results can be precise but not accurate, if every reading is wrong by the same amount.
The resolution of an instrument is the smallest change it can show. A ruler marked in mm has a resolution of 1 mm.

Random and systematic errors

Random errors make readings scatter unpredictably above and below the true value, for example starting a stopwatch a little early or late. Reduce their effect by repeating readings and calculating a mean.
Systematic errors make every reading wrong by the same amount in the same direction, for example a balance that reads 0.3 g with nothing on it (a zero error). Repeating does not help. Check and reset the instrument, or correct every reading.

Evaluating an experiment

To evaluate, ask: Were the results repeatable? Were there anomalies? What errors could there be?
Then suggest specific improvements: an instrument with a better resolution, more repeats, a data logger instead of a stopwatch, or controlling a variable better.
Read scales at eye level to avoid reading them at an angle.
Worked example

A balance reads 0.3 g with nothing on it. A sample then reads 25.4 g. What is the true mass?

  1. Every reading is 0.3 g too high (a zero error).
  2. Subtract it: 25.4 − 0.3

Answer: 25.1 g

Key idea

Accurate means close to the true value; precise means repeat readings agree. Random errors scatter results (repeat and take a mean). Systematic errors shift every reading the same way (fix or correct the instrument).

Key words
Mean
An average: add up the readings and divide by how many there are.
Repeatable
Results are repeatable if the same person using the same method and kit gets similar results again.

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Common mistakes
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Extra practice

2 more questions

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  1. The true length of a rod is 50.0 cm. Which set of readings is precise but NOT accurate?

    1. 40, 52, 61 cm
    2. 49.9, 50.0, 50.1 cm
    3. 47.1, 47.2, 47.1 cm
    4. 45, 50, 55 cm
    Show the answer

    Answer: 47.1, 47.2, 47.1 cm

    Precise readings are close together; accurate ones are close to the true value.

  2. A balance reads 0.5 g with nothing on it. What kind of error does this cause?

    1. Systematic error
    2. No error
    3. Random error
    4. An anomaly
    Show the answer

    Answer: Systematic error

    A zero error shifts every reading.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.