4. Parallax Error
4.1. Examples
Reading a ruler from an angle so the measurement appears shorter or longer than the true length
Viewing a thermometer from below eye level, causing the meniscus to appear higher than it actually is
Reading an analogue ammeter or voltmeter from the side, so the pointer appears displaced from its true position on the scale
Reading a measuring cylinder from above, making the meniscus appear higher than its true level
Viewing a burette from an angle during a titration, giving a consistently incorrect volume reading
Parallax Error: A Four-Step Analysis
Use the four-step framework to analyse parallax error:
- Step 1 — Identify the source
Observation / procedure — the researcher’s eye is not level with the scale or pointer, causing an apparent shift in the reading.
- Step 2 — Classify the behaviour
Consistent, one-direction shift → Systematic error → affects accuracy. If the observer’s eye is always in the same wrong position, every reading is displaced in the same direction by a similar amount.
- Step 3 — Explain the impact
All results are shifted consistently too high or too low. The error cannot be detected by repeating measurements, as each repeat is made from the same incorrect viewing angle. Precision is unaffected — results will agree with each other — but none will be close to the true value.
- Step 4 — Suggest an improvement
Parallax error is eliminated, not averaged out — position the eye correctly before every reading.
4.2. Effects
Parallax error produces a consistent offset across all measurements. Because the observer’s eye is in the same wrong position for every reading:
results will appear precise (repeats agree with each other),
but accuracy is reduced — all values are displaced from the true value in the same direction,
repeating measurements does not help, as the error affects every reading equally,
the direction of the error depends on viewing angle: reading from below gives a value that is too high; reading from above gives a value that is too low.
4.3. Improvements
To eliminate parallax error, position the eye correctly and use instruments designed to minimise the effect.
Position the eye directly level with the scale, pointer, or meniscus before taking every reading.
For liquid levels, read at the bottom of the meniscus with the eye at the same height as the liquid surface.
Use instruments with a mirror scale (common on analogue meters) — align the pointer with its reflection before reading to confirm the eye is directly in front.
Use digital instruments where possible, as they eliminate the need to interpret a pointer or meniscus position.
Mark a consistent eye-level reference point when repeated readings are taken at the same position.
Structured Question: Parallax Error
A student is investigating the relationship between the volume of sodium hydroxide solution and the temperature change produced when it is added to a fixed volume of hydrochloric acid. The student uses a burette to measure the volume of sodium hydroxide delivered in each trial. Throughout the experiment, the student reads the burette from slightly below eye level.
(a) Identify the type of error introduced by the student’s reading technique and classify it as random, systematic, or personal. (2 marks)
(b) Explain how this error would affect the student’s volume measurements. In your answer, refer to the direction of the error and its effect on the accuracy and precision of the results. (3 marks)
(c) The student repeats each volume measurement three times and calculates a mean. Evaluate whether this would reduce the effect of the error identified in part (a). (2 marks)
(d) Describe one improvement the student could make to eliminate this error. (1 mark)
Reveal Answer Key
(a)
The error is a parallax error, classified as a systematic error.
(1 mark for naming parallax error; 1 mark for systematic)
(b)
Because the student’s eye is consistently positioned below the level of the meniscus, the meniscus appears higher on the burette scale than its true position. This means the volume reading will be consistently overestimated — every reading is displaced in the same direction by a similar amount. (1 mark)
The accuracy of the results is reduced, as all recorded volumes are higher than the true volume delivered. (1 mark)
The precision is unaffected — because the eye is in the same wrong position for every reading, the repeated measurements will agree closely with each other. (1 mark)
(c)
Repeating measurements and calculating a mean would not reduce the effect of this error. (1 mark)
Because the eye is in the same incorrect position for every reading, each repeat is displaced by the same amount in the same direction. Averaging does not cancel a consistent offset — it only reduces the effect of random errors, which vary unpredictably between trials. (1 mark)
(d)
The student should position their eye directly level with the meniscus before taking each reading, ensuring the line of sight is horizontal and perpendicular to the burette scale. (1 mark)
Part (b): award marks only if the student correctly identifies the direction (overestimate, since the eye is below the meniscus) and correctly distinguishes between accuracy (reduced) and precision (unaffected). A response that states precision is also reduced should not receive the precision mark.
Part (c): a response that simply states “repeating reduces error” without explaining why it does not apply to systematic error should not receive full marks.
Part (d): accept “use a digital burette” or “use a burette with a mirror scale and align the pointer with its reflection” as alternative valid responses.