[BioC] Limma final gene expression report

Tue May 10 13:21:15 CEST 2005

Dear Ankit,

In addition to Mick's arguments, you also need to be careful when averaging 
over spots if you use the empirical bayes methods in limma, and you have 
different number of replicates for different genes. If, for example, gene A 
has 1 spot but gene B has four replicated spots, then the variance of the 
mean of the replicates is very different for gene B and for gene A, and would 
violate some of the assumptions behind the Empirical Bayes procedure in 
limma. I think this is mentioned in the user guide (and/or Gordon Smyth's 
paper), and has also been mentioned in this list.

If you really want an average, you can do as Mick suggests:
> > If you *really* want an average value for each spot,
> > simply take the average M value from the output of
> > toTapble.

you can use something like

tapply(the.top.table.M.values, the.top.table.gene.identifiers, mean)

Best,

R.

On Tuesday 10 May 2005 11:59, Ankit Pal wrote:
> Dear Mick,
> Thanks a lot for the reply.
> I am interested in the spots individually but for
> further analysis of the spots I need a single
> representative value for each gene.
> I have looked up the manual, I did not find a way to
> combine replicate spots into a single value.
> Could you tell me what is the method or which section
> of the manual is it present in.
> t will be of great help to me.
> Thank you,
> -Ankit
>
>
> --- "michael watson (IAH-C)"
>
> <michael.watson at bbsrc.ac.uk> wrote:
> > There are ways of combining replicate spots in
> > limma, and it is all in the user guide :-)
> >
> > However, many people, myself included, prefer things
> > reported on a spot-by-spot basis.  If all replicate
> > spots for a particular gene are reported as
> > significant, I take that as further proof that i)
> > the gene is differentially expressed, ii) my arrays
> > are of good quality, iii) my experimental procedure
> > was of good quality.  Think about the case where
> > only one out of two spots is reported - is that
> > because one of the spots was of poor quality?  Or
> > because the values for each spot differ by a lot?
> > You would lose this valuable information if you just
> > took the average between replicates.
> >
> > If you *really* want an average value for each spot,
> > simply take the average M value from the output of
> > toTapble.
> >
> > Mick
> >
> >
> > -----Original Message-----
> > From: bioconductor-bounces at stat.math.ethz.ch on
> > behalf of Ankit Pal
> > Sent: Tue 10/05/2005 6:15 AM
> > To: bioconductor at stat.math.ethz.ch
> > Cc:
> > Subject: [BioC] Limma final gene expression report
> >
> > Dear All,
> > While looking at the Limma user guide, I came across
> > the following example
> >
> > > targets <- readTargets("SwirlSample.txt")
> > > RG <- read.maimages(targets$FileName,
> >
> > source="spot")
> >
> > > RG$genes <- readGAL()
> > > RG$printer <- getLayout(RG$genes)
> > > MA <- normalizeWithinArrays(RG)
> > > MA <- normalizeBetweenArrays(MA)
> > > fit <- lmFit(MA, design=c(-1,1,-1,1))
> > > fit <- eBayes(fit)
> > > options(digits=3)
> > > topTable(fit, n=30, adjust="fdr")
> >
> > ID        Name       M    A     t  P.Value    B
> > control   BMP2      -2.21 12.1 -21.1 0.000357 7.96
> > control   BMP2      -2.30 13.1 -20.3 0.000357 7.78
> > control   Dlx3      -2.18 13.3 -20.0 0.000357 7.71
> > control   Dlx3      -2.18 13.5 -19.6 0.000357 7.62
> > fb94h06 20-L12       1.27 12.0  14.1 0.002067 5.78
> > fb40h07  7-D14       1.35 13.8  13.5 0.002067 5.54
> >
> > I have omitted a few rows and columns.
> > Here we see that after all the data transformations,
> > we get an output where the ranking for the probes in
> > an array is  done on the basis of the B value.
> > Notice that there are reapeating names for genes,
> > therefore for a set of replicates, within and across
> > arrays, each spot is reported separately as an
> > individual entity.
> > In the case of BMP2 from the above example, which
> > result do I consider?
> > Is there a way in which I can get a single result
> > for
> > a set of replicates.
> > I am new to this package, so please do let me know
> > if
> > there is a problem in my understanding the concept.
> > Thank you,
> > -Ankit
> >
> > _______________________________________________
> > Bioconductor mailing list
> > Bioconductor at stat.math.ethz.ch
> > https://stat.ethz.ch/mailman/listinfo/bioconductor
>
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-- 
Ramón Díaz-Uriarte
Bioinformatics Unit
Centro Nacional de Investigaciones Oncológicas (CNIO)
(Spanish National Cancer Center)
Melchor Fernández Almagro, 3
28029 Madrid (Spain)
Fax: +-34-91-224-6972
Phone: +-34-91-224-6900

http://ligarto.org/rdiaz
PGP KeyID: 0xE89B3462
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