Systematic error
Measurements & Uncertainties - OCR A-Level Physics
Key Definition
Systematic error
An error that shifts all readings by the same amount in the same direction. Cannot be reduced by repeating measurements.
An error that shifts all readings by the same amount in the same direction. Cannot be reduced by repeating measurements.
- Systematic errors make results consistently too high or too low.
- Examples: zero error on a balance, a ruler with a worn end, background radiation not subtracted.
- A zero error means the instrument does not read zero when it should.
- Systematic errors affect accuracyHow close a measurement is to the true or accepted value. High accuracyHow close a measurement is to the true or accepted value. High accuracy means low systematic error. means low systematic errorAn error that shifts all readings by the same amount in the same direction. Cannot be reduced by repeating measurements.. but NOT precisionHow close repeated measurements are to each other. A precise set of results has a small spread (low random error)..
- Fix by recalibrating the instrument or correcting for the offset.
Key Definition
Random error
An error that causes readings to scatter unpredictably above and below the true value. Can be reduced by averaging repeated measurements.
An error that causes readings to scatter unpredictably above and below the true value. Can be reduced by averaging repeated measurements.
- Random errors cause readings to vary each time you measure.
- Examples: timing human reaction delay, reading a scale from slightly different angles (parallax error), thermal fluctuations.
- Random errors affect precisionHow close repeated measurements are to each other. A precise set of results has a small spread (low random errorAn error that causes readings to scatter unpredictably above and below the true value. Can be reduced by averaging repeated measurements.)..
- Reduce by taking more repeats and calculating the mean.
- Also reduce by using instruments with finer resolutionThe smallest change in a quantity that an instrument can detect. For example, a ruler has a resolution of 1 mm. (e.g. vernier caliper instead of ruler).
Common Mistake
MEDIUM
Wrong: Saying 'take more repeats' will fix a systematic errorAn error that shifts all readings by the same amount in the same direction. Cannot be reduced by repeating measurements..
Right: Repeats only reduce random errorAn error that causes readings to scatter unpredictably above and below the true value. Can be reduced by averaging repeated measurements.. Systematic errors shift every reading the same way, so averaging does NOT remove them. You must recalibrate or use a different method.
Right: Repeats only reduce random errorAn error that causes readings to scatter unpredictably above and below the true value. Can be reduced by averaging repeated measurements.. Systematic errors shift every reading the same way, so averaging does NOT remove them. You must recalibrate or use a different method.