[BioC] Affy: Present calls in an eset

A.J. Rossini rossini at blindglobe.net
Wed Oct 8 08:05:56 MEST 2003

<Arne.Muller at aventis.com> writes:

> I get your point with interpreting absent/present calls. Technically it's a
> nice feature, becasue one can just discard the majority of the genes on the
> chip for further analysis. In fact I think absent/present calls make sense in
> terms of biology, since just a fraction of the genes are realy expressed at a
> time. How to express this numerially is a different story (and I guess a
> difficult one).

Sure, but equivalently, I could implement the "feature" with "Tony's
Complete Crud Calls" (TCCC):


Change the 20% expressed to a different number if it makes you feel

I'm not being quite fair, but from my perspective, it's only slightly
less realistic than the MAS or Resolver calls -- the difference being
that the probability of response is an unclear function of the
expression level, with a trend where the probabilty of the call goes
up with the expression level (which is the technical component).

And then, you've got the possibility of low levels of expression which
are real but hard to pick out.

> Anyway, with MAS the calls are calculated anyway, can't they? So, I'd be nice
> (at least for "completness") to add a "mascall" method to the exprSet objects
> generated by affy. What do you think?

It would be great.  As far as I know, the MAS calls are protected as a
trade secret and non-disclosure agreements.  Maybe not, but I happen
to know for sure that the rosetta resolver calls are! (or I'd have
implemented them for amusement)

> By the way, if you ignore the call, do you set an arbitrary intensity cutoff
> later in your analysis, or do just reley on the statistics (anova  p-value or
> whatever)?

If you have biological replicates, then either, depending on what you
are looking for.  I've never found an approach which works all the
time.  If you have technical replicates, then the calls are necessary,
but note that they don't have biological meaning, just technical
(technology) meaning.


rossini at u.washington.edu            http://www.analytics.washington.edu/ 
Biomedical and Health Informatics   University of Washington
Biostatistics, SCHARP/HVTN          Fred Hutchinson Cancer Research Center
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