Tries the effect of adding and dropping individual terms from a `REML`

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

### Options

`PRINT` = string tokens |
Controls printed output (`changes` ); default `chan` |

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

`CHANGES` = pointer |
Saves details of the changes |

### Parameter

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

### Description

VRTRY 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 `VRSWITCH`

.) 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 any 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. However, `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 `VRFI`

T procedure should also be called before `VRTRY`

is used. However, `VRTRY`

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 `VRTRY`

(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 `TRY`

directive. Any term that is not already in the current model is added to the model. The effect of the change is recorded, and then the term is taken back out of the model. Conversely, any term that is already in the current model is dropped from the model. Again, the effect of the change is recorded, before the term is added back into the model. 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.

By default `VRTRY`

prints a table showing the effect of adding and dropping the various terms. However, you can suppress that by setting option `PRINT=*`

.

You can use the `CHANGES`

option to save information about the changes in a pointer. The first elements of the pointer, labelled ‘`Change`

‘, ‘`s.s`

.’, ‘`n.d.f.`

‘ and ‘`d.d.f`

.’, save the corresponding columns of the table in a text (‘`Change`

‘) and three variates. The final element, ‘`term`

‘, is a pointer storing a model formula for the term associated with each change. The final line of the table (Residual of initial model) is not included. The pointer is defined so that the case of the labels is not significant.

Options: `PRINT`

, `FACTORIAL`

, `CHANGES`

.

Parameter: `TERMS`

.

### Action with `RESTRICT`

Any restriction applied to vectors used in the `REML`

analysis will apply also to the results from `VRTRY`

.

### See also

Directives: `FIT`

, `REML`

.

Procedures: `VRFIT`

, `VRADD`

, `VRDISPLAY, VRDROP`

, `VRKEEP`

, `VRSETUP`

, `VRTRY`

.

Commands for: REML analysis of linear mixed models.

### Example

CAPTION 'VRTRY 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] VRTRY [PRINT=changes] Littersize+Sex+Dose