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 (or
differences) 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 s to 10.3 s

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 all of the numbers:
= 50.5
There are 5 items, so divide by 5:
Mean (or average) = =
= 10.1 s
Mean (or average) = sum of values
total # of values

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 and will respond to smaller changes in temperature.

12. Definitions : Precision
As well as accuracy, precision is important.
Precision is how close the measured values are to each other.
It may be connected to the smallest scale division on your measuring instrument.

13. Definitions : Precision
For example, using a ruler:
A ruler with a millimeter scale
will give greater precision than a ruler with a centimeter scale.
Your measurement is more precise.

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 pencil is weighed on A, 3 times:
The readings are: 7.3 g, 7.7 g, 7.1 g A
So the Range is
= 7.1 g – 7.7 g = 0.6 g
It is then weighed on B, 3 times:
The readings are: 7.5 g, 7.3 g, 7.4 g
So the Range is B
= 7.3 g – 7.5 g = 0.2 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.