Statistical analysis for A2 psychology

Choosing a statistical test

There are several factors that must be considered before a statistical test can be chosen. Firstly the research design, aim and level of measurement must be identified:

**Data can be either related or unrelated.**

Research design

Research design

**Related data**is produced from

**repeated measures**and

**matched pairs**designs.

**Unrelated data**is produced from

**independent groups**designs.

**Research aim**

Is the aim of the research to investigate a significant difference or a significant association? If, for example, there are 2 groups of participants, each in a different condition of the independent variable (e.g. in an experiment), then the aim is to test for a

**significant differenc**e. If the aim is to test for a correlation between two variables, then the aim is to test for a

**significant association**.

**Level of measurement**

Data can be produced at nominal, ordinal and interval levels:

**Nominal data**is the most basic level of measurement. An example is a frequency count of a distinct category, such as the number of aggressive and non-aggressive acts in an observation.**Ordinal data**consists of a list of data that can be ranked in order, but not data that would fit to an interval scale. An example is the subjective rating of happiness (on a scale form 1 to 10) that participants may score themselves as on a questionnaire. A happiness rating of 10 is higher than 5, but it is not twice as happy as 5 or 5 times as happy as 2.**Interval data**is measured on a scale in which each interval is exactly the same size. Time is interval data because each second is the same duration, and 10 seconds are twice as long as 5 seconds.

Statistical tests

Once the design, aim and level of measurement have been identified, the correct inferential test can be chosen.

**Spearman’s rho is a test for significant association, and produces a correlation coefficient. The level of measurement must be either ordinal or interval. The research design can be either related or unrelated.**

Spearman’s rho

Spearman’s rho

**Wilcoxon signed ranks test**

Wilcoxon signed ranks is a test of significant difference for related data. The research design must produce related data (e.g. repeated measures or matched pairs). The level of measurement can be either ordinal or interval.

**Mann-Whitney U test**

Mann-Whitney U is a test of significant difference for unrelated data. The research design must produce unrelated data (e.g. independent measures). The level of measurement can be either ordinal or interval.

**Chi-square test**

Chi-square tests for difference when the data is nominal and unrelated. The research design must produce unrelated data (e.g. independent measures). The level of measurement must be nominal (e.g. categories).

A Level exam tips

Answering exam questions (AQA specification)

Questions about inferential statistics require you to be able to identify the correct test for a given research example or data set. You will not be asked to actually calculate the test.