Forcing integers to be nominal - R-help

Wed, Aug 7, 2002 12:10 AM #

Hi everyone,

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:

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?

-Tim

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Brian Ripley

Wed, Aug 7, 2002 12:17 AM #

On Wed, 7 Aug 2002, Tim Wilson wrote:

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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Torsten Hothorn

Wed, Aug 7, 2002 12:30 AM #

see ?(as.)factor

Torsten

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John Fox

Wed, Aug 7, 2002 4:51 AM #

Dear Tim,

As has been pointed out, you need to ensure that YearColl is coerced to a 
factor. The read.spss function by default converts variables to factors 
when they have value labels in the SPSS data set; otherwise numeric 
variables without value labels remain numeric. See help(read.spss).

I hope that this helps,
  John

At 02:10 AM 8/7/2002 -0500, Tim Wilson wrote:

-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox
-----------------------------------------------------

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