Transformations for repeated-measures t-test
Account for non-normal distributions when comparing two observations of a continuous outcome
1. Researchers can identify any outliers (values that are more than 3.29 standard deviations away from the mean) for each observation of the outcome and make sure that the values were entered correctly. If there were, and the amount of outliers does not make up more than 10% of all observations, then researchers can delete the outliers in a "listwise" fashion. This means that the observation is deleted outright. This is the much less preferable to option two below.
The steps for conducting a Wilcoxon test in SPSS
2. Click Analyze.
3. Drag the cursor over the Nonparametric Tests drop-down menu.
4. Drag the cursor over the Legacy Dialogs drop-down menu.
5. Click 2 Related Samples.
6. Click on the first observation of the ordinal outcome.
7. Click on the arrow button to move the first observation of the outcome variable into the Test Pairs box under the Variable1 column.
8. Click on the second observation of the ordinal outcome.
9. Click on the arrow button to move the second observation of the outcome variable into the Test Pairs box under the Variable2 column.
10. Click OK.
The steps for interpreting the SPSS output for a Wilcoxon test
If it is LESS THAN .05, then researchers have evidence of a statistically significant effect in the ordinal outcome across time or within-subjects.
If the p-value is MORE THAN .05, then researchers have evidence that there is NOT a statistically significant effect in the ordinal outcome across time or within-subjects.
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