Question: F-tests for factorial effects - limma
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14.3 years ago by
Naomi Altman6.0k
Naomi Altman6.0k wrote:
I am analyzing a 2-factor factorial Affy experiment, with 3 d.f. for each factor. I would like to get the F-tests for the main effects and interactions using limma. I have computed all the contrasts, and got the t-tests (both unadjusted and eBayes). I do know how to combine these into F-tests "by hand" but I could not figure out if there was a simple way to do this using limma. I had a look at FStat (classifyTestsF). There seems to be a problem, in that the matrix tstat is not premultiplied by the contrast matrix when the F-statistics are computed. So, if the contrasts are not full-rank, an error is generated (instead of the F-statistics) because nrow(Q) != ncol(tstat).. (See the lines below). if (fstat.only) { fstat <- drop((tstat%*% Q)^2 %*% array(1, c(r, 1))) attr(fstat, "df1") <- r attr(fstat, "df2") <- df return(fstat) } I figured that before I fiddled with the code, I would check to make sure that I didn't miss an existing routine to do this. Thanks in advance. Naomi S. Altman 814-865-3791 (voice) Associate Professor Bioinformatics Consulting Center Dept. of Statistics 814-863-7114 (fax) Penn State University 814-865-1348 (Statistics) University Park, PA 16802-2111
affy limma • 495 views
modified 14.3 years ago by Gordon Smyth36k • written 14.3 years ago by Naomi Altman6.0k
Answer: F-tests for factorial effects - limma
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14.3 years ago by
Gordon Smyth36k
Walter and Eliza Hall Institute of Medical Research, Melbourne, Australia
Gordon Smyth36k wrote:
> Date: Tue, 21 Dec 2004 17:21:19 -0500 > From: Naomi Altman <naomi@stat.psu.edu> > Subject: [BioC] F-tests for factorial effects - limma > To: bioconductor@stat.math.ethz.ch > > I am analyzing a 2-factor factorial Affy experiment, with 3 d.f. for each > factor. > > I would like to get the F-tests for the main effects and interactions using > limma. > > I have computed all the contrasts, and got the t-tests (both unadjusted and > eBayes). I do know how to combine these into F-tests "by hand" but I could > not figure out if there was a simple way to do this using limma. limma doesn't have any easy way to deal with main effects and interactions, at least not with main effects, interactions are actually simpler. I haven't implemented this because I've never been able to figure out what one would do with these things in a microarray context. To compute F-tests for main effects and interaction, the easiest way would probably be to compute the SS for main effects and interactions by non-limma means, then use shrinkVar() to adjust the residual mean squares, i.e., the F-statistic denominators. If you only want F-tests for interactions, the following code would work: X <- model.matrix(~a*b) fit <- lmFit(eset, X) p <- ncol(X) cont.ia <- diag(p)[,attr(X,"assign")==3] fit.ia <- eBayes(contrasts.fit(fit, cont.ia)) Now fit.ia contains the F-statistic and p-values for the interaction in fit.ia$F and fit.ia$F.p.value. > I had a look at FStat (classifyTestsF). There seems to be a problem, in > that the matrix tstat is not premultiplied by the contrast matrix when the > F-statistics are computed. So, if the contrasts are not full-rank, an > error is generated (instead of the F-statistics) because nrow(Q) != > ncol(tstat).. (See the lines below). No, the code is correct. FStat is quite happy with non full rank contrasts but the contrast matrix must be applied using contrasts.fit() before entering FStat(). You should not expect to see a contrast matrix inside the classifyTestsF() code. Gordon > if (fstat.only) { > fstat <- drop((tstat%*% Q)^2 %*% array(1, c(r, 1))) > attr(fstat, "df1") <- r > attr(fstat, "df2") <- df > return(fstat) > } > > I figured that before I fiddled with the code, I would check to make sure > that I didn't miss an existing routine to do this. > > Thanks in advance. > > Naomi S. Altman 814-865-3791 (voice) > Associate Professor > Bioinformatics Consulting Center > Dept. of Statistics 814-863-7114 (fax) > Penn State University 814-865-1348 (Statistics) > University Park, PA 16802-2111