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