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VRDROP procedure

Drops terms in a REML fixed model from a Genstat regression (R.W. Payne).


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


TERMSformula Fixed terms to be dropped

VRDROP 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, VRADDVRDISPLAY, VRKEEPVRSETUPVRSWITCH 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.

The terms to be dropped are specified by the TERMS parameter, in a similar way to the FIT directive. 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.

Parameter: TERMS.

Action with RESTRICT

Any restriction applied to vectors used in the REML analysis will apply also to the results from VRADD.

See also

Directives: FIT, REML.
Commands for: REML analysis of linear mixed models.


CAPTION     'VRDROP example',\
            'Example 5.3.6 from The Guide to Genstat, Part 2 Statistics';\
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; \
CLOSE       chan
VCOMPONENTS [FIXED=Littersize+Dose*Sex] RANDOM=Dam/Pup
REML        [PRINT=model,components,wald] Weight
VRFIT       [PRINT=estimates] Littersize+Sex*Dose
VRDROP      [PRINT=estimates,accumulated] Sex.Dose
Updated on October 29, 2020

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