Thank you for your help professor Ripley!

I suppose my problem is more theoretical related than syntax. I will review
your suggested literature.

and yes, I did error on the data source, thank you for catching my mistake!!

Best,

Dave

On Sat, Jan 9, 2010 at 3:56 AM, Prof Brian Ripley <ripley@stats.ox.ac.uk>wrote:

> What are you trying to do?  Your example is not what is commonly called
> ANOVA (some call it ANCOVA) and more often lm() is used.
>
> I suspect that you intended 'population' to be a factor, and it is not.  So
> population:condition is not an interaction but different slopes for
> population by levels of condition.
>
> population*condition expands to
>
> population + condition + population:condition
>
> and this is a larger model with different intercepts by levels of
> condition.
>
> I suggest you need study the primary reference (Chambers & Hastie 1992) or
> at least Bill Venables' exposition in MASS (the book, any edition).  And
> note that you cannot test interactions in a two-way layout without
> replication, so perhaps you also need to talk to a statistician about ANOVA.
>
> BTW: I think you have messed up your first example: perhaps you meant
>
> studentDataSource="http://files.davidderiso.com/r/studentData.data"
>
> There are no P values in that example because there is no residual
> variation: the model fits exactly.
>
>
> On Sat, 9 Jan 2010, Dave Deriso wrote:
>
>  Hello,
>>
>> I have a simple question about using the aov function syntax (ie. * + or
>> :)
>> for the interaction of 2 factors. I have read the help files, and
>> researched
>> other sites, and have included my source files. My goal is to measure the
>> signifigance of the interaction between population and condition (aka.
>> population:condition). I can't seem to figure it out.
>>
>> 1. In the first example the significance of population:condition works
>> with
>> the "allData" but not with the "studentData." Can you please explain why
>> it
>> fails and how I can fix it?
>>
>> 2. In the second example I can get the measure the significance of
>> population:condition with 2 different methods, but I get 2 different
>> results
>> (using the "allData" source). Can you please explain why these Pr(>F)
>> values
>> are different?
>>
>> Thank you so much for your help!!
>>
>> Sincerely,
>>
>> Dave Deriso
>> UCSD Psychiatry
>>
>>
>> #Example 1 ---------------------------COPY & PASTE THE FOLLOWING
>>
>> #import the data
>> allDataSource="http://files.davidderiso.com/r/allData.data"
>> allData.import=read.table(allDataSource,header=T)
>> studentDataSource="http://files.davidderiso.com/r/allData.data"
>> studentData.import=read.table(studentDataSource,header=T)
>>
>>
>> #aov for allData WORKS
>> allData.integral.aov = aov(integral~population*condition,
>> data=allData.import)
>> summary(allData.integral.aov)
>>
>> #aov for studentData DOES NOT GIVE Pr(>F) of population:condition
>> studentData.integral.aov = aov(integral~population*condition,
>> data=studentData.import)
>> summary(studentData.integral.aov)
>>
>>
>>
>> #Example 2 ---------------------------COPY & PASTE THE FOLLOWING
>>
>> #population:condition has a Pr(>F) of 0.96372
>> allData.integral.aov = aov(integral~population*condition,
>> data=allData.import)
>> summary(allData.integral.aov)
>>
>> #population:condition has a Pr(>F) of 1.070e-06 ***
>> allData.integral.aov = aov(integral~population:condition,
>> data=allData.import)
>> summary(allData.integral.aov)
>>
>>        [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> R-help@r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>>
> --
> Brian D. Ripley,                  ripley@stats.ox.ac.uk
> Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/<http://www.stats.ox.ac.uk/%7Eripley/>
> University of Oxford,             Tel:  +44 1865 272861 (self)
> 1 South Parks Road,                     +44 1865 272866 (PA)
> Oxford OX1 3TG, UK                Fax:  +44 1865 272595
>

	[[alternative HTML version deleted]]

