To ensure that your results are valid, consider the following guidelines when you collect data, perform the analysis, and interpret your results.
 The data should include at least 1 random factor.
 If you do not have any random factors, use Fit General Linear Model. For more information on random factors, go to What is the difference between fixed and random factors?.
 The response variable should be continuous
 If the response variable is categorical, your model is less likely to meet the assumptions of the analysis, to accurately describe your data, or to make useful predictions.
 The sample data should be selected randomly

Random samples are used to make generalizations, or inferences, about a population. If your data were not collected randomly, your results might not represent the population.
 Collect data using best practices

To ensure that your results are valid, consider the following guidelines:
 Make certain that the data represent the population of interest.
 Collect enough data to provide the necessary precision.
 Measure variables as accurately and precisely as possible.
 Record the data in the order it is collected.
 The model should provide a good fit to the data

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.