[BioC] help for BH - FDR and Fold Change filtering question,

James MacDonald jmacdon at med.umich.edu
Fri Sep 21 15:24:56 CEST 2007

Hi Saurin,

Saurin D. Jani wrote:
> Hi BioC,
> For affymetrix microarrays, I have two groups: WT (wild type) and KO (knock out). I normalize data by some method, and then I do following steps for all my GeneChips.
> - Perform t-test 
> - generate p-value
> - get Fold Change 
> - compute adjusted p-value (BH method)
> Now, I do filter genes based on two options, please let me know which is better.
> OptionsA:
> - filter genes which satisfies p-value and fold change threshold and 
> get number genes and get max. adjusted p-value. as FDR.
> OptionsB:
> - filter genes which satisfies BH-FDR (fdr x) and I get N number of genes, so, then I do Fold Change threshold so, I get N-1 genes and  report N-1 genes with x fdr. 
> is my x fdr is still true for N-1 genes or no? because have done extra step after I filter genes based on fdr. 

No, the fdr will not be accurate any more.



> Thank you so much in advance,
> Saurin
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James W. MacDonald, MS
UMCCC cDNA and Affymetrix Core
University of Michigan
1500 E Medical Center Drive
7410 CCGC
Ann Arbor MI 48109

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