[R] Within Subject ANOVA question

Mike Lawrence Mike.Lawrence at dal.ca
Tue Jun 2 01:08:43 CEST 2009


On Mon, Jun 1, 2009 at 7:50 PM, tsunhin wong <thjwong at gmail.com> wrote:
> But I have some follow-up questions:
> 1. In aov, why IVs (subject & condition) cannot be in number format?

Put simply, because that's the only way to obtain the values you
observe in the paper you're reading. :Op  Factorizing tells the ANOVA
that the labels associated with each level of the factor aren't really
meaningful. This is particularly evident in the case of Subject
numbers, which (typically) have no numeric interpretation (Ss #6 is
not 2 times Ss #3). The original authors' choice to treat condition as
a factor was a less clear choice given that it clearly refers to a
measurable ratio quantity (seconds). I presume the paper to which
you're referring is the Masson & Loftus 1994 paper (be sure to read
their 2003 follow-up and 2004 correction). In that case the authors
come from a field where it's commonplace to treat even ratio
independent variables as factored.

> 2. Why I need to use factor() on IVs (subject & condition) but I
> cannot use factor() on DV (recall)?

As noted above, factoring prohibits R from interpreting the labels of
a value as numerically meaningful, something you surely do not want to
do with your DV in this example.


-- 
Mike Lawrence
Graduate Student
Department of Psychology
Dalhousie University

Looking to arrange a meeting? Check my public calendar:
http://tr.im/mikes_public_calendar

~ Certainty is folly... I think. ~




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