Log fold change filter using limma

Question

[flute28]

This is probably a stupid question, but I felt the need to ask it as I couldn't seem to find the answer in the limma documentation. But if you wanted to filter some microarray data with a log fold change of 1.5% , what would you enter into the limma topTable function. Does the lfc parameter take percentages directly i.e should I enter lfc=1.5 or should I enter lfc=0.015.

[russhh]

I'm not quite sure what you mean by 1.5% with respect to log-fold changes in microarray analysis. Do you want the genes that display log fold-changes in the top 1.5% of all genes studied? or do you want genes where the fitted parameter for log-fold-change is at least 1.5% of the constant coefficient in your model

[WouterDeCoster]

Log fold change is generally not presented with a %, where did you find that?

[Mike]

This is not presented in term of percentage, simply fold change is a ratio between two conditions, and its normalised by log2.

for example:

logFC =   log2 (expression value of treatment / expression value of control)
     eg.  expression value of treatment = 16.2512
             expression value of control = 1.202, 
                = 16.2512/1.202 = 13.5201; 
                =log2(13.5201)
                so, logFC = 3.75

its nicely explained by Devon Ryan’s post:

R: How to convert log2FC (Fold Change) obtained by limma's topTable() function to FC

see below link also

http://rstudio-pubs-static.s3.amazonaws.com/13988_bb11d85b79b2436280de434988558140.html