To ensure that your results are valid, consider the following guidelines when you collect data, perform the analysis, and interpret your results.
For more information on factors, go to Factors and factor levels and Fixed and random factors.
If the sample size is greater than 15 or 20, the test performs very well with skewed and nonnormal distributions. If the sample size is less than 15 or 20, the results might be misleading with nonnormal distributions.
If you are not confident that your data follow a normal distribution and you do not meet the sample size guidelines, use Kruskal-Wallis Test.
If you have dependent observations, go to Analyzing a repeated measures design. For more information about samples, go to What are independent samples?.
If the model does not fit the data, the results can be misleading. In the output, use the residual plots, the diagnostic statistics for unusual observations, and the model summary statistics to determine how well the model fits the data.