Use the residual plots to help you determine whether the model is adequate and meets the assumptions of the analysis. If the assumptions are not met, the model may not fit the data well and you should use caution when you interpret the results.
For more information on how to handle patterns in the residual plots, go to Residual plots for Analyze Factorial Design and click the name of the residual plot in the list at the top of the page.
Residuals versus fits plot
The patterns in the following table may indicate that the model does not meet the model assumptions.
||What the pattern may indicate
|Fanning or uneven spreading of residuals across fitted values
||A missing higher-order term
|A point that is far away from zero
|A point that is far away from the other points in the x-direction
||An influential point
Use the residuals versus fits plot to verify the assumption that the residuals are randomly distributed and have constant variance. Ideally, the points should fall randomly on both sides of 0, with no recognizable patterns in the points.
Residuals versus order plot
Use the residuals versus order plot to verify the assumption that the residuals are independent from one another. Independent residuals show no trends or patterns when displayed in time order. Patterns in the points may indicate that residuals near each other may be correlated, and thus, not independent. Ideally, the residuals on the plot should fall randomly around the center line: