This menu allows you to fit the variance shift outlier model (VSOM) to detect outliers in a REML analysis.

## Residual term to check for outliers

The dropdown list contains the residual strata in the REML model. Select the strata that is to be checked for outliers.

## Display

This controls what output is printed from the analysis:

Outliers |
A list of possible outliers |

False discovery rate |
The false discovery rates for the potential outliers, which are estimated from the distribution of p-values calculated from the t-statistics using an asymptotic model, See the FDRMIXTURE procedure for more information on the estimation of false discovery rates |

## Plot

This controls what plots are displayed from the analysis:

Index plots |
The estimated statistics from the analysis plotted against their index (order) in the data set |

Residual plot |
The usual residual plot with outlying points coloured by their outlier status |

### Components in the index plot

The index plot can have three components plotted:

Omega |
Variance shift as a ratio to the residual variance |

Sigma squared |
Estimated residual variance under VSOM |

Statistic |
The statistic selected in the Methods section |

The graph can be given a **Title** by entering the text in that field. If no title is given, then default titles will be used. Use a single space in the field to suppress titles.

## Methods

This controls what plots are displayed from the analysis:

### Calculating statistics

The deviance associated with a variance shift for a particular unit in the strata can be calculated using 3 statistics:

t |
Uses the squared t-statistics (i.e. squared standardized residuals) to approximate the change in likelihood (default); this is the fastest approach |

Partial likelihood |
The partial likelihood which approximates the change in likelihood, where the baseline model parameters are held fixed, and only the extra variance component for each unit is estimated; this is much faster than re-estimating the full model |

Full likelihood |
The full likelihood calculated by refitting the full model with the added variance term for each unit; this can be very time-consuming |

### Constrain variance components to be positive

This constrains the variance components in the model, including the variance shift for the units, to be positive.

### Calculating thresholds

The thresholds used to decide if an outlier is significant at the 5%, 1% or 0.1% level can be calculated two ways:

Bootstrap |
Uses parametric bootstrap samples, with the variance components in the baseline model, to calculate the thresholds from the percentiles of the order statistics |

Approximate |
Uses the asymptotic distribution to calculate the thresholds |

## Number of samples

This option specifies the number of bootstrap samples that are performed, default 499 (as well as the “null” model where the data keep their original values).

## Seed

Specifies the seed for the random number generator used to make the bootstrap samples; default 0 continues from the previous generation or (if none) initializes the seed automatically.

## Save

Use these options to save results from the VSOM analysis. After selecting the **Results** box, you need to type the identifier of a pointer into the **In**: field. The elements in the results pointer are given in the VSOM help.

### Display in spreadsheet

The saved results will be displayed in a new spreadsheet.

## See also

- VSOM procedure to calculate the variance shift outlier model.
- VPLOT procedure for plotting residuals.
- FDRMIXTURE procedure.
- Linear Mixed Model menu.
- Initial Values for specifying initial gamma.
- Further Output for obtaining additional output after fitting a model.
- Save for saving the results from a REML analysis.
- Save REML results in a spreadsheet.
- Residual Plots for generating plots of residuals.
- Means Plots for generating plots of one- or two-way tables of means.
- REML directive for command mode use of REML, with additional options to

control the algorithm and for more sophisticated analyses. - VCOMPONENTS directive for further information about fixed, random, and spline model terms.
- Linear Mixed Models (REML) – Correlated Errors for setting up covariance models.
- REML Power calculations dialog for calculating the power of a REML analysis.
- REML Permutation Test dialog for calculating the significance of a random term using a permutation test.