[BioC] RMA normalization and MAS5.0 detection calls

James W. MacDonald jmacdon at med.umich.edu
Thu Dec 7 15:35:06 CET 2006

Hi Haiyan,

haiyan wu wrote:
> Hi,all
> I'm using Bioconductor for analyze some Affymetrix Genechip.When I use RMA
> to normalize probe sets,it give no info for whether the probe sets present
> or absent.
> So I  get these  info from MAS5 detection calls.But in some case,the DE
> probes sets which was selected seems absent .On other situation for contrast
> different treatment,
> some probe sets presnt in treatment1 and absent in treatment2, but limma
> give me a conclusion that this probe sets have no changed between these 2
> treatment.
> How can I solve this problem? Is it right only using RMA value for limma and
> igore present/absent calls?

There are many ways to approach an analysis, and I don't think there is 
any objective way to determine which is the best way. Some people do 
just what you have done, computing expression values using RMA or GCRMA 
and then using P/A calls to filter out those they think are not expressed.

The rationale for doing this is that the MM probes give a reasonable 
estimate of background for the majority of the probes on a given chip, 
so if there is no statistical difference between PM and MM, then you 
might be able to consider that gene unexpressed.

However, RMA does not make use of the MM probes at all, and GCRMA only 
uses the MM data in aggregate (rather than a probe-by-probe fashion), so 
it is not surprising that you get the results you mention for some 

Another way to approach filtering probesets is based on the variability 
of the probesets over all samples. If the variance is low (below some 
constant c), then you might assume that the gene is not differentially 
expressed in any samples (which is different than saying it is expressed 
or not). These genes are uninteresting by definition, and can be removed 
from the dataset.



> Regards!
> haiyan
> 	[[alternative HTML version deleted]]
> _______________________________________________
> Bioconductor mailing list
> Bioconductor at stat.math.ethz.ch
> https://stat.ethz.ch/mailman/listinfo/bioconductor
> Search the archives: http://news.gmane.org/gmane.science.biology.informatics.conductor

James W. MacDonald, M.S.
Affymetrix and cDNA Microarray Core
University of Michigan Cancer Center
1500 E. Medical Center Drive
7410 CCGC
Ann Arbor MI 48109

Electronic Mail is not secure, may not be read every day, and should not be used for urgent or sensitive issues.

More information about the Bioconductor mailing list