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 <rip...@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, rip...@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]] ______________________________________________ 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.
