Precision and accuracy: why repeats aren't everything

Science · Scientific Inquiry · ages 11-12

Name ______________________ Date ____________

You need to measure exactly 5 ml of liquid. Which instrument should you use?
- A 1 litre measuring jug
- A 10 ml measuring cylinder
- A kitchen bucket
You need to measure out 5 ml of water for an experiment. Which instrument is the best choice?
- A 10 ml measuring cylinder
- A 1-litre measuring jug
- A bathroom scale
- A metre ruler
A measurement is accurate when it is...
- close to the true value
- close to your other readings
- written with many digits
- taken very quickly
What does it mean for a set of measurements to be precise?
- They are all close to each other
- They are all close to the true value
- They are written in standard form
A balance measures a mass as 14.63 g, but it can only be trusted to 3 significant figures. Round the reading to the correct number of significant figures, in g.
Answer: ______________
A stopwatch shows 12.68 seconds. Round this time to 3 significant figures.
Answer: ______________
How many significant figures does the measurement 0.0502 m have?
- 3
- 5
- 4
- 1
Round 4.736 to 3 significant figures.
Answer: ______________
Zara weighs the same stone five times and gets very similar readings, but every one of them is 1 g heavier than a trusted lab scale's reading. What should Zara do?
- Weigh the stone many more times so the readings average out
- Check whether her scale is zeroed correctly before weighing again
- Write her answer to more significant figures
A scientist needs to measure 3 ml of a chemical. She has a 100 ml measuring cylinder and a 5 ml measuring cylinder. Which should she pick, and why?
- Either one, because both give the same accuracy
- The 100 ml cylinder, because it can hold more liquid
- The 5 ml cylinder, because its scale divisions are small enough to read 3 ml accurately

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 · The 10 ml cylinder has finer scale divisions, so you can read 5 ml far more precisely than on a jug or bucket.
A 10 ml measuring cylinder · The 10 ml measuring cylinder is made for small volumes and has fine marks, so 5 ml can be measured with much less uncertainty than with a big jug.
close to the true value · Accuracy is about being close to the true value. Being close to your other readings is precision, which is a different idea.
They are all close to each other · Precision is about repeats agreeing with each other. It doesn't tell you whether they match the true value, that's accuracy.
14.6 · 14.63 has four significant figures. Rounding to three gives 14.6 g.
12.7 · The first three significant figures are 1, 2, and 6. The next digit is 8, which is 5 or more, so round the 6 up to 7: 12.7 seconds.
3 · Zeros at the start of a decimal do not count. The significant figures are 5, 0, and 2, so there are 3.
4.74 · Keep the first three significant digits (4, 7, 3) and look at the next digit, 6. Since 6 is 5 or more, round the 3 up to 4, giving 4.74.
Check whether her scale is zeroed correctly before weighing again · Repeating the measurement only checks precision. Since every reading is wrong in the same direction, the problem is with the scale itself, not random variation.
The 5 ml cylinder, because its scale divisions are small enough to read 3 ml accurately · A bigger cylinder has wider scale divisions, so small volumes like 3 ml are harder to read precisely. The 5 ml cylinder's finer scale suits the job.