To conduct a repeated measures one-way ANOVA in SPSS,
go to 'Analyze', 'General Linear Model', 'Repeated Measures'.
Create a name for the variable you want to define;
for example 'conditie (condition)',
and then indicate the number of levels this variable contains.
In this case, that number is 3 (measurements 1, 2, and 3), add them.
Click 'Define'. The variable 'conditie (condition)' has been defined, but its levels have not been entered.
Select the first measurement ('meting1') for level 1,
the second measurement ('meting2') for level 2,
and the third measurement ('meting3') for level 3.
Under 'Contrasts', you can define the contrasts for this repeated measure.
For example, using the 'Helmert' contrast.
Click 'Change' and 'Continue'.
Under 'Plots', you can plot the main effect for 'conditie (condition)'. Click 'Add' and 'Continue'.
The post hoc analyses are available under 'Options'.
Display the main effect here and tick 'Compare main effects', e.g. using 'Sidak'.
We also want a homogeneity test and maybe some descriptive statistics
and anything else you might be interested in. Click 'Continue', 'Paste', and run the syntax.