[BioC] Background correction (Normexp+offset)

Kachroo, Priyanka priya_coll at neo.tamu.edu
Tue Jul 12 17:40:26 CEST 2011

Dear All,

 I needed your help with some 2-color microarray data analysis. So the problem is that after sorting by pvalue and fold change cut off of 1.5, I am left with very few differentially expressed genes. I use Normexp method for background correction with an offset value of 50 (default).

1. So if i use offset=50, i get downregulated genes=11, upregulated genes=31

2. If i use offset=25, i get downregulated=14 , upregulated=46

3. If i use offset=10, i get downregulated=20 ,upregulated=93

I read on the Limma-bioconductor forum that making a boxplot of foreground and background (green and red channels) should help decide if background correction is needed or not. I made that boxplot but do not know how to interpret it. I could not attach the MA plots for the offset values 10 and 25  with this email. Can someone guide me as to how to interpret MA plots after background correction and what offset values to use. 

Also this is what the moderator writes for a way to decide the offset value " You can judge a good value for the offset by inspection of the MA-plots. If you really want a quantitative way to judge this, look at the component fit$df.prior after you use the eBayes() function in limma. The better you stabilize the variances, the larger will be df.prior and the greater will be the power to detect DE genes. Hence the offset which maximises df.prior is, in sense, optimal "

So, when i run my code and type fit$df.prior i get a value of 1.481457. How does this number help me decide the offset.

Priyanka Kachroo
Graduate Assistant Research
Texas A&M University

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