Find definitions and interpretation guidance for every residual plot.

The histogram of the residuals shows the distribution of the residuals for all observations.

Use the histogram of the residuals to determine whether the data are skewed or include outliers. The patterns in the following table may indicate that the model does not meet the model assumptions.

Pattern | What the pattern may indicate |
---|---|

A long tail in one direction | Skewness |

A bar that is far away from the other bars | An outlier |

Because the appearance of a histogram depends on the number of intervals used to group the data, don't use a histogram to assess the normality of the residuals.

A histogram is most effective when you have approximately 20 or more data points. If the sample is too small, then each bar on the histogram does not contain enough data points to reliably show skewness or outliers.

The residuals versus fits graph plots the residuals on the y-axis and the fitted values on the x-axis.

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.

The patterns in the following table may indicate that the model does not meet the model assumptions.

Pattern | What the pattern may indicate |
---|---|

Fanning or uneven spreading of residuals across fitted values | Nonconstant variance |

Curvilinear | A missing higher-order term |

A point that is far away from zero | An outlier |

A point that is far away from the other points in the x-direction | An influential point |

The following graphs show an outlier and a violation of the assumption that the variance of the residuals is constant.

If you identify outliers in your residual versus fits plot, consider the following solutions:

- Verify that the observation is not a measurement error or data-entry error.
- Consider performing the analysis without this observation to determine how it impacts your results.

The residuals versus order plot displays the residuals in the order that the data were collected.

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:

If you see a pattern, investigate the cause. The following types of patterns may indicate that the residuals are dependent.

The residuals versus variables plot displays the residuals versus another variable. The variable could already be included in your model. Or, the variable may not be in the model, but you suspect it affects the response.

If the variable is already included in the model, use the plot to determine whether you should add a higher-order term of the variable. If the variable is not already included in the model, use the plot to determine whether the variable is affecting the response in a systematic way.

These patterns can identify an important variable or term.

Pattern | What the pattern may indicate |
---|---|

Pattern in residuals | The variable affects the response in a systematic way. If the variable is not in your model, include a term for that variable and refit the model. |

Curvature in the points | A higher-order term of the variable should be included in the model. For example, a curved pattern indicates that you should add a squared term. |