Adds or drops terms from a `REML`

fixed model in a Genstat regression (R.W. Payne).

### Options

`PRINT` = string tokens |
Controls printed output (`model` , `deviance` , `summary` , `estimates` , `correlations` , `fittedvalues` , `accumulated` ); default `mode` , `summ` , `esti` , `accu` |

`FACTORIAL` = scalar |
Limit for expansion of terms; default 3 |

`DENOMINATOR` = string token |
Whether to base ratios in accumulated summary on rms from model with smallest residual ss or smallest residual ms (`ss` , `ms` ); default `ss` |

`SELECTION` = string tokens |
One or two criteria to be printed with the models (`%variance` , `%ss` , `adjustedr2` , `r2` , `dispersion` , `aic` , `sic` , `bic` ); default `%var` , `aic` , `sic` |

### Parameter

`TERMS` = formula |
Fixed terms to be added or dropped |

### Description

`VRSWITCH`

is one of several procedures designed to improve the process of determining the appropriate fixed terms to include in a `REML`

analysis. (The others are `VRFIT`

, `VRADD`

, `VRDISPLAY`

, `VRDROP`

`VRKEEP`

, `VRSETUP`

and `VRTRY`

.) They do this by a generalized regression analysis, with a weight matrix based on variances estimated from the original `REML`

analysis (with the full fixed model). See `VRFIT`

for details.

Before fitting the terms, the `VRSETUP`

procedure must be called to make some checks, and initialize the regression by specifying a `MODEL`

command with the necessary weight matrix and a `TERMS`

command with the full fixed model. It also uses the `WORKSPACE`

directive to set up a Genstat workspace structure to store control information and results. `VRFIT`

will call `VRSETUP`

for you, if you have not done so already. The analysis will then be based on the most recent `REML`

analysis. To use an earlier analysis, you should call `VRSETUP`

yourself, setting its SAVE option set to the save structure of the required `REML`

analysis.

In principle the `VRFIT`

procedure should also be called before `VRSWITCH`

is used. However, `VRSWITCH`

will call `VRFIT`

with a null model (i.e. only the constant) if `VRFIT`

has not been used already. So you can start investigating the fixed model just by calling `VRSWITCH`

(and `VRFIT`

and `VRSETUP`

will be called for you, automatically).

The `TERMS`

parameter specifies the terms to be added or dropped, in a similar way to the `SWITCH `

directive. Any term that is not already in the current model is added to the model. Conversely, those that are already in the model are dropped. The `FACTORIAL`

option sets a limit (by default 3) on the number of factors and variates in each term. Terms containing more than that number are omitted.

The `PRINT`

option controls printed output as in the regression directives, except that some irrelevant settings are omitted. (For example, `grid`

is relevant only to the fitting of nonlinear models.) See `VRDISPLAY`

for more details.

The `DENOMINATOR`

option specifies how the residual is selected for the accumulated analysis of variance By default it is taken from the model with the smallest number of residual degrees of freedom. However, you can set `DENOMINATOR=ms`

to take it from the model with the smallest residual mean square.

The `SELECTION`

option specifies the statistics to be displayed in the summary of analysis as in the regression directives, except that again some irrelevant settings are omitted. See `VRDISPLAY`

for more details.

Options: `PRINT`

, `FACTORIAL`

, `DENOMINATOR`

, `SELECTION`

.

Parameters: `TERMS`

.

### Action with `RESTRICT`

Any restriction applied to vectors used in the `REML`

analysis will apply also to the results from `VRSWITCH`

.

### See also

Directives: `FIT`

, `REML`

.

Procedures: `VRFIT`

,`VRADD`

, `VRDISPLAY`

, `VRDROP`

, `VRKEEP`

, `VRSETUP`

, `VRTRY`

.

Commands for: REML analysis of linear mixed models.

### Example

CAPTION 'VRSWITCH example',\ 'Example 5.3.6 from The Guide to Genstat, Part 2 Statistics';\ STYLE=meta,plain FACTOR [NVALUES=322; LEVELS=27] Dam & [NVALUES=322; LEVELS=18] Pup FACTOR [NVALUES=322; LEVELS=2; LABELS=!T('M','F')] Sex FACTOR [NVALUES=322; LEVELS=3; LABELS=!T('C','Low','High')] Dose VARIATE [NVALUES=322] Littersize,Weight OPEN '%GENDIR%/Examples/GuidePart2/Rats.dat'; CHANNEL=chan READ [CHANNEL=chan] Dose,Sex,Littersize,Dam,Pup,Weight; \ FREPRESENTATION=2(labels),4(levels) CLOSE chan VCOMPONENTS [FIXED=Littersize+Dose*Sex] RANDOM=Dam/Pup REML [PRINT=model,components,wald] Weight VRFIT [PRINT=estimates] Littersize+Sex VRSWITCH [PRINT=estimates,accumulated] Sex+Dose