The Sign Test
What is the Sign Test?
- The sign test is a method used in interferential statistics to determine whether or not an observed result (from an experiment) is significant or not
- It is a non-parametric test which means that there is no assumption that the data will follow a normal distribution
- It is known as the Sign Test as it is based on the number of plus or minus signs present in the data after the calculations have taken place
What criteria determine the use of the Sign Test?:
- investigating a difference (i.e. an experiment)
- repeated measures design (i.e. each participant experiences all conditions of the IV)
- nominal data (i.e. data in categories)
- Is the hypothesis directional or non-directional (this will determine which critical value to apply to the data)
Advantages of using the Sign Test
- It is a simple test which is easy to carry out
- It can be applied across a range of situations where a normal distribution cannot be assumed
Disadvantages of using the Sign Test
- Nominal data is the least powerful type of data which means that the Sign Test can sometimes be unreliable
- It may not be suitable for use with small samples or when the median has been used as the measure of central tendency
An abnormal distribution: one of the criteria for using the Sign Test
Worked example
LEVEL: 3 MARKS
A researcher hypothesised that exercising before taking a memory test would significantly improve memory. These are the results showing the memory test scores per participant:
|
Participant |
Exercise before test |
No exercise |
Difference |
Sign of difference |
|
1 |
15 |
9 |
6 |
|
|
2 |
7 |
12 |
-5 |
|
|
3 |
18 |
3 |
15 |
|
|
4 |
5 |
5 |
0 |
|
|
5 |
11 |
12 |
-1 |
|
|
6 |
9 |
17 |
-8 |
|
|
7 |
13 |
8 |
5 |
|
|
8 |
6 |
16 |
-10 |
|
|
9 |
10 |
14 |
-4 |
|
12.1 Using the data in the table above carry out the Sign Test and calculate the value of S.
S = [3]
AO2 = 3 marks
For full marks the answer should correctly carry out the Sign Test: subtract ‘No exercise’ from ‘Exercise before test’ and add each score to the ‘Difference’ column. Add a + or – according to whether each difference is positive or negative then count up the number of +s and –s. The lower of the two scores gives the S value. In this case the S value is 3 (as there are 3 positive signs).
