Repeat a measurement and the numbers rarely match exactly. SPM Science expects you to find the mean, describe the spread, and explain the differences as variation rather than a fault.
This lesson follows reading supplied temperature and pulse data in the body-parameter cluster.
Why do repeated readings differ?
Three things cause the differences: small counting slips, small changes in the person from one minute to the next, and the limits of the instrument.
None of these means the experiment failed. They mean one reading should not carry the whole conclusion.
Worked example: four resting pulse readings
Siti counts her resting pulse four times, in beats per minute.
| Reading | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Beats per minute | 74 | 76 | 72 | 90 |
Step 1: the mean of all four. (74 + 76 + 72 + 90) ÷ 4 = 312 ÷ 4 = 78 beats per minute.
Step 2: look for a reading that does not fit. Reading 4 is 14 above the next highest value. Siti notes she had just climbed the stairs before that count.
Step 3: the mean without reading 4. (74 + 76 + 72) ÷ 3 = 222 ÷ 3 = 74 beats per minute.
Step 4: write the conclusion. The resting pulse is about 74 beats per minute. Reading 4 was excluded because Siti was not at rest, and the range of the remaining readings is 76 − 72 = 4.
The reason to drop a reading must come from the situation, not from the number looking awkward.
The mistake that costs marks
Students delete a reading because it is far from the others and give no reason. The mean looks cleaner, but the answer has no scientific support.
| Wrong | Right | |
|---|---|---|
| Reading 90 | “Removed, it is wrong” | Removed because the measurement was taken after stairs |
| Mean stated | 74 | 74, with the reason and the range |
| Reason given | none | yes, from the situation |
If there is no known reason to drop a reading, keep it and report the mean of all readings, noting the wide spread.
Check yourself
Four temperature readings in °C are 36.6, 36.8, 36.7 and 36.7. Find the mean and the range.
Answer
Sum: 36.6 + 36.8 + 36.7 + 36.7 = 146.8. Mean: 146.8 ÷ 4 = 36.7 °C.
Range: 36.8 − 36.6 = 0.2 °C. The readings are close together, so the variation is small.
What to study next
The next lesson, distinguishing curriculum calculations from personal health assessment, shows what your mean does and does not allow you to say. Practise judging fair comparisons with the graph evidence comparison lab.
For a teacher to rehearse the wording with you, see online one-to-one Science tuition.