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Precision and accuracy: why repeats aren't everything

Science · Scientific Inquiry · ages 11-12

Name ______________________ Date ____________

You need to measure 5 ml of water. All these tools are correctly calibrated. Which has the most suitable scale?
- A 1 litre measuring jug with divisions every 100 ml
- A 10 ml measuring cylinder with divisions every 0.5 ml
- A 2 litre measuring container with divisions every 500 ml
A water-filled measuring cylinder stands upright on a level table. Where should your eye be when reading the bottom of the curved water surface?
- Above the water surface, looking down into the cylinder.
- Below the water surface, looking up through the cylinder.
- Level with the cylinder base, looking across the table surface.
- Level with the water surface, looking straight across it.
A trusted reference mass is 30.0 g. Which reading is closest to that reference?
- 29.9 g
- 29.2 g
- 31.3 g
- 32.1 g
What describes precise repeat readings taken under the same conditions?
- They agree closely with one another.
- They always equal the accepted reference value.
- They show every digit produced by a calculator.
Round the reported mass 14.63 g to 3 significant figures. Enter the rounded value in grams.
Answer: ______________
How many significant figures are reported in the measurement 7.080 cm?
Answer: ______________
Noah counts all five digits in 0.0502 m as significant figures. How many should he count?
- 5
- 4
- 3
- 2
Round 0.004762 m to 2 significant figures. Enter the rounded value in metres.
Answer: ______________
Zara's repeat mass readings agree closely, but each is 1 g above a trusted reference measurement. What should she do next?
- Take many more repeats and assume their mean will remove the shared 1 g offset.
- Check the zero setting, equipment and method against a known mass before repeating.
- Write many more decimal places and assume those digits will remove the shared offset.
A cylinder has finer volume divisions than another cylinder. Its calibration and reading method have not been checked. What can be concluded from the divisions alone?
- Every reading will be exact, so reference checks and careful reading are unnecessary.
- Every reading will be accurate, even if its zero setting and reading method have faults.
- The finer scale helps reading detail, but does not establish the readings' accuracy.

Answer key

For grown-ups. Fold this page away before handing over the rest.

Precision and accuracy: why repeats aren't everything W1-mt_auJqTemMpI-s1

A 10 ml measuring cylinder with divisions every 0.5 ml · The 10 ml cylinder holds 5 ml and shows smaller volume divisions than the other tools.
Level with the water surface, looking straight across it. · Read the bottom of water's curved surface at eye level. Looking from above or below can change the apparent reading.
29.9 g · 29.9 g is 0.1 g from the reference. The other differences are 0.8 g, 1.3 g and 2.1 g.
They agree closely with one another. · Precision describes agreement between repeats. Accuracy concerns closeness to a reference.
14.6 · Keep 1, 4 and 6. The next digit is 3, so the rounded mass is 14.6 g.
4 · Count 7, the middle 0, 8 and the final decimal 0. There are four significant figures.
3 · The zeros before 5 are placeholders. Count 5, the middle 0 and 2: three significant figures.
0.0048 · Ignore the leading zeros. Keep 4 and 7. The next digit is 6, so round the 7 up to 8: 0.0048 m.
Check the zero setting, equipment and method against a known mass before repeating. · A shared offset needs investigation. Repeats or extra digits do not remove it, and the pattern alone does not identify its cause.
The finer scale helps reading detail, but does not establish the readings' accuracy. · Finer divisions can reduce reading uncertainty. Calibration and method still affect accuracy.