Quick Summary
- Take 3–5 repeat readings per value to increase precision and reliability, then average them.
- Use a wide range (5–10 values) with equal step sizes of 1, 2, 5, or a multiple of 10 — this reveals the true pattern in your data.
- Record data to the resolution of your instrument, and generally quote final answers to 3 significant figures.
- Spot anomalous readings (differ by more than ~10% from the mean) — ignore them when averaging, and repeat that measurement.
- A micrometer reads to 0.01 mm using a main scale (barrel) + thimble scale — always tighten with the ratchet, never the barrel.
- Reduce timing errors by measuring over multiple oscillations and using a fiducial marker at the point of highest speed.
- Use set squares and plumb lines to check that apparatus is truly vertical, horizontal, or parallel before taking measurements.
1. Number of Readings
Here's the thing about a single measurement: it could be wrong, and you'd never know. Maybe your reaction time was off, maybe there was a random gust of wind, maybe you misread the scale by a hair. A single reading has no way of checking itself.
That's why we take repeat readings — usually 3 to 5 times for the same value of the independent variable. Then we average them. Averaging repeats does three jobs at once:
- Increases precision — random errors tend to cancel out when you average, so your value clusters closer to the "true" one.
- Increases confidence — if your three readings are all close together, you know it wasn't a fluke.
- Checks reproducibility — if repeats scatter wildly, that tells you something is unstable in your method, and you'd want to investigate before trusting the data.
Rule — Uncertainty from Repeats
Absolute uncertainty in repeat readings = ± ½ × (range of readings)
In plain English: take your biggest reading, subtract your smallest, then halve it. That's your ± value.
Recording to the right precision
Every instrument has a resolution — the smallest change it can actually detect. Your recorded values must match that resolution consistently. For example:
- A micrometer (resolution 0.01 mm) → record to 2 decimal places, e.g. 1.40 mm, not 1.4 mm.
- A ruler measuring in cm (resolution 0.1 cm) → record to 1 decimal place, e.g. 5.3 cm.
- A ruler measuring in metres (resolution 0.01 m / 1 mm) → record to 2 decimal places, e.g. 0.35 m.
Beyond raw resolution, final calculated answers are usually quoted to 3 significant figures — this is generally the level of precision you can confidently read off a graph or trust in a calculation.
💡 Examiner Tip
Never just say "take readings of the voltage" in a plan — that's too vague to gain marks. Instead say something like: "take 10 readings of voltage between 0.5 V and 5 V in steps of 0.5 V, each with 3 repeats, and calculate a mean." Specific numbers = marks.
Practice Question
2. Range of Measurements
reliable
different
range
5 to 10 values
1, 2, 5, or a multiple of 10
Why range matters — the diode example
A narrow range can completely hide the real physics.