[R] Forcing integers to be nominal

[ripley@stats.ox.ac.uk]

On Wed, 7 Aug 2002, Tim Wilson wrote:

> I've got a problem with an analysis of variance where it appears that my
> independent variable is being treated as an integer when it should be
> nominal.
>
> The data are being loaded from an SPSS file and the independent variable
> 'YearColl' corresponds (surprisingly enough) to a student's year in
> college. :-) The integers 1-4 are used in the dataset for this purpose.
> My quick little anova gives this result:
>
> > mir.aov <- aov(DepressT ~ YearColl, data=mir)
> > summary(mir.aov)
>              Df  Sum Sq Mean Sq F value   Pr(>F)
> YearColl      1   955.7   955.7  10.305 0.001474 **
> Residuals   293 27172.5    92.7
> ---
> Signif. codes:  0 `***' 0.001 `**' 0.01 `*' 0.05 `.' 0.1 ` ' 1
>
> I was expecting the df for YearColl to be (k-1) = 3.
>
> Am I reading this right? Is there a way to force integers to act like
> nominal data?

What you mean is that you want YearColl to be a factor and so coded.
as.factor() will help.

R has no idea what you meant: you gave it an integer variable, and it
handled it as such.  Don't be surprised if R does as it is told.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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