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Testing for Normality using SPSS (cont...)

Procedure when there are two or more independent variables

The Explore... command on its own cannot separate the dependent variable into groups based on not one but two or more independent variables. However, we can perform this feat by using the Split File... command.

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SPSS

Output

You will now see that the output has been split into separate sections based on the combination of groups of the two independent variables. As an example we show the tests of normality when the dependent variable, "Int_Politics", is categorized into the first "Gender" group (male) and first "Edu_Level" group (School). All other possible combinations are also presented in the full output but we will not shown them here for clarity.

Normality Screenshot

Published with written permission from SPSS Inc., an IBM Company.

Under this above category you are presented with the Tests of Normality table as shown below:

Normality Screenshot

Published with written permission from SPSS Inc., an IBM Company.

The Shapiro-Wilk test is now analysing the normality of "Int_Politics" on the data of those individuals that are classified as both "male" in the independent variable, "Gender", and "school" in the independent variable "Edu_Level". As the Sig. value under the Shapiro-Wilk column is greater than 0.05, we can conclude that "Int_Politics" for this particular subset of individuals is normally distributed.

The same data from the same individuals are now also being analysed to produce a Normal Q-Q Plot as below. From this graph, we can conclude that the data appears to be normally distributed as it follows the diagonal line closely and does not appear to have a non-linear pattern.

Normality Screenshot

Published with written permission from SPSS Inc., an IBM Company.

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