2. Copy these terms into your exercise book with the correct definition Accuracy Precision (of data) Reliability Results are close to the true value Repeat results are grouped around a similar value Improved by repetition of results or comparison with someone else who has done the same experiment

3. Accuracy When doing an experiment we want to find the true value This is an accurate value

4. Precision Precision is how closely our data points are groups Ideally we want our data to be both precise and accurate

5. Precision without accuracy It is possible your results are very precise but totally inaccurate E.g. A length of a block of wood is actually 42 cm. A student takes 3 readings of the length and gets 20.1 cm, 19.8 cm, 20.0 cm. The results are totally off the accurate value but still very precise. What might have caused this?

6. Accuracy without Precision By a happy coincidence, we may have very imprecise results but when we calculate the mean we get an accurate value. E.g. for the same block of wood which is 42 cm, we get measurements of 29 cm, 52 cm, 44 cm. They vary widely but have a mean value of 42 cm!

7. Accuracy compared with Precision 0 true value 0 0 Precise (grouped) but not accurate. Accurate (the mean) but not precise. Accurate and Precise.

8. Reliability Reliable results are ones where repeats have been made It gives us an idea of the precision of the data Highlights anomalies Taking a mean should get us close to an accurate value

9. Reliability Reliable data is evidence you can trust. If someone else did the same experiment, they would get the same result. Your evidence will be more reliable if you repeat your readings. For example…

10. Reliability For example: 3 students measure the time for 1 swing of a pendulum: Discuss which method is the most reliable, and why. Jo measures 1 swing. Emma measures 1 swing, but 20 times, and calculates the average (mean) time. Jack measures 20 swings and divides the time by 20. Physics for You page 359