[BioC] Mean or median centering data

[kumar s]

Dear all, 
I have time course data (processed and have expression
values as log2 fold changes) with and without drug at
various time points. 

I want to do a clustering experiment with data. 

I read from various papers that before doing
clustering, the data is normalized using mean, median
or centroid mean. 

I have 2 questions:

1. Why do I have to do normalization using mean or
median or centroid mean. what is going to happen if I
standardize. what is specifically centroid mean and
how it differs from mean and median.


2. How can I do it in R. are there any packages
/functions available. 

thank you very much. 

ps




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