[BioC] MLSeq Mathematical Concepts

[Wolfgang Huber]

Dear Dario

good points, and as usual in machine learning, I don’t expect there to be a simple answer or universally best solution.
For classification, the (pre)selection  of features (genes) used is probably more important than most other choices, esp. if the classification task is simple and can be driven by a few genes. For clustering, similar, plus the choice of distance metric or embedding.

That said, it is plausible that both, using the untransformed counts (or RPKMs etc.), or the log-transformed values, have problems with high variance (either at the upper or lower end of the dynamic range) that can be avoided with a different transformation, log-like for high values, linear-like for low (e.g. DESeq2’s vst, rlog). Paul McMurdie and Susan Holmes have some on this in their waste-not-want-not paper [1], and Mike in a Supplement to the DESeq2 paper (draft). It would be interesting to collect more examples, and someone should probably study this more systematically (if they aren’t already.)

Kind regards
	Wolfgang


[1] http://www.ploscompbiol.org/article/info%3Adoi%2F10.1371%2Fjournal.pcbi.1003531
[2 http://www-huber.embl.de/DESeq2paper —> Regularized logarithm for sample clustering (As of today, there is a version of 19 February which I think will soon be updated with a more extensive survey).




Il giorno 23 Apr 2014, alle ore 07:00, Dario Strbenac <dstr7320 at uni.sydney.edu.au> ha scritto:

> Hello,
> 
>> From reading the vignette, MLSeq seems to be a set of wrapper functions that allows the user easy access to normalisation strategies in edgeR or DEseq and passes the data onto algorithms such as Support Vector Machine or Random Forest. Are there any results that demonstrate that normalisation improves classification performance ? I am also not convinced about the description of using voom weights to transform the data. The author of voom stated that specialised clustering and classification algorithms are needed to handle the CPM and weights separately. Why does MLSeq use standard classification algorithms and how were the weights and expression values combined ?
> 
> --------------------------------------
> Dario Strbenac
> PhD Student
> University of Sydney
> Camperdown NSW 2050
> Australia
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