1. How Science works:
Taking measurements

2. Learning Objectives
You should learn :
About taking measurements,
The meaning of ‘variation’, ‘range’ and ‘mean (average)’,
The meaning of ‘accuracy’ and ‘precision’.

3. Taking measurements When you take measurements
there may be some variation in the readings.
For example:
If you time the fall of a paper parachute over a fixed distance, the times may vary slightly.
10.1 s, s, s, s, s
Let’s look at these results more closely.

4. Taking measurements The results were:
10.1 s, s, s, s, s
What is the Range of these results?

5. Taking measurements : Range
The results were:
10.1 s, s, s, s, s
Find the minimum value
and the maximum value
Range
= from min to max
= 9.9 to 10.3

6. Taking measurements : Mean
The results were:
10.1 s, s, s, s, s
What is the mean (or average) of these results?

7. = 50.5 5 Taking measurements : Mean The results were:
10.1 s, s, s, s, s
Add up the 5 numbers:
= 50.5
There are 5 items, so divide by 5:
Mean (or average) = =
= 10.1 s

8. Taking measurements : Mean
The results were:
10.1 s, s, s, s, s
Why is it a good idea to calculate the mean of your results?
Because it improves the reliability of your results.
Your results will be more reliable.

9. Accuracy
and
Precision

10. Accuracy and Precision …sound the same thing…
Definitions
Accuracy and Precision …sound the same thing…
…is there a difference??

11. Definitions : Accuracy
In your experiments, you need to consider the accuracy of your measuring instrument.
For example:
An expensive thermometer is likely to be more accurate than a cheap one.
It will give a result nearer to the true value.
It is also likely to be more sensitive (with a better resolution). It will respond to smaller changes in temperature.

12. Definitions : Precision
As well as accuracy, precision is also important.
Precision is connected to the smallest scale division on the measuring instrument that you are using.
For example:

13. Definitions : Precision
For example, using a ruler:
A ruler with a millimetre scale
will give greater precision than a ruler with a centimetre scale.

14. Definitions : Precision
A precise instrument also gives a consistent reading when it is used repeatedly for the same measurements.
For example:

15. Definitions : Precision
For example, 2 balances:
A beaker is weighed on A, 3 times:
The readings are: 73 g, 77 g, 71 g A
So the Range is
= 71 g – 77 g = 6 g
It is then weighed on B, 3 times:
The readings are: 75 g, 73 g, 74 g
So the Range is B
= 73 g – 75 g = 2 g
Balance B has better precision.
Its readings are grouped closer together.

16. Accuracy compared with Precision
Suppose you are measuring the length of a wooden bar:
true
value
The length has a true value
And we can take measurements of the length, like this:
Let’s look at 3 cases…

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