[BioC] Re: technical replicates (again!): a summary

Gordon Smyth smyth at wehi.edu.au
Thu Apr 1 02:29:37 CEST 2004

Dear Ramon,

Thanks for your constructive post. I'm sorry my replies will have to be too 

At 01:49 AM 1/04/2004, Ramon Diaz-Uriarte wrote:
>Dear Gordon, Naomi, and BioC list,
>The issue of how to deal with technical replicates (such as those obtained
>when we do dye-swaps of the same biological samples in cDNA arrays) has come
>up in the BioC list several times. What follows is a short summary, with
>links to mails in BioC plus some questions/comments.
>There seem to be three major ways of approaching the issue:

There is another practical approach which is to fit the technical replicate 
as a fixed effect rather than as a random effect. See Naomi's addition to 
the discussion summary. This works when the number of levels is not too 
large and there are a respectable number of replicates per level. It's not 
the absolutely ideal solution, but it can be good for people with the right 
sort of data who want to something here and now with existing software.

>1. Treat the technical replicates as ordinary replicates
>E.g., Gordon Smyth in sept. 2003
>However, this makes me (and Naomi Altman ---e.g.,
>uneasy (tech. reps. are not independent biological reps. which leads to the
>usual inflation of dfs and deflation of se).
>I guess part of the key to Gordon's suggestion is his comment that even if 
>s.e. are slightly underestimated, the ordering is close to being the optimal
>one. But I don't see why the ordering out to be much worse if we use the
>means of technical replicates as in 3. below. (Haven't done the math, but it
>seems that, specially in the pressence of strong tech. reps. covariance and
>small number of independent samples we ought to be better of using the means
>of the tech. reps).

See comment below. There are some situations where I think this is still 
the best method within the limitations of existing Bioconductor software, 
especially when the number of independent samples is small.

>2. Mixed effects models with subject as random effect (e.g., via lme).
>In late August of 2003 I asked about these issues, and Gordon seemed to agree
>that trying the lme approach could be a way to go.
>However, in September, I posted an aswer and included code that, at least for
>some cases, shows potential problems with using lme when the number of
>technical replicates is small.
>Nevertheless, Naomi Altman reports using lme/mixed models in a couple of
>After reading about randomizedBlock (package statmod) in a message in BioC (I
>think from Gordon), I have tried aggain the mixed models approach, since with
>tech. reps and no other random effects, we should be able to use
>randomizedBlock. Details in 5. below:

I think that fitting any statistical model, random effects or otherwise, to 
each gene in a microarray is risky in terms of false discovery rate without 
some sort of moderation across the genes. See comment below.

>3. Take the average of the technical replicates
>Except for being possibly conservative (and not estimating tech. reps.
>variance component), I think this is a "safe" procedure.
>This is what I have ended up doing routinely after my disappointing tries 
>and what Bill Kenworthy seemed to end up doing
>I think this is also what is done at least some times in literature (e.g.,
>Huber et al., 2002, Bioinformatics, 18, S96--S104 [the vsn paper]).

Two comments. Firstly, this strategy only works when you have a balanced 
design and no missing values. It would be hard for me to recommend it as a 
universal strategy because people send me emails all the time with half of 
their data missing.

Secondly, this is "safe" in the usual statistical sense of ensuring the 
size of your test, for an individual gene, is not larger than the nominal 
size. But you are worrying only about bias when noise is also an issue. In 
the microarray context, it can pay in terms of overall false discovery rate 
to introduce bias into estimation of the variances if it makes the 
variances more stable.

>4. Dealing with replicates in future versions of limma
>Now, in Sept. 2004 Gordon mentioned that an explicit treatment of tech. reps.
>would be in a future version of limma
>and I wonder if Gordon meant via mixed-effects models, or some other way, or
>if there has been some progress in this area.

I and a student have done quite a bit of work in this direction, but I'm 
not ready yet to put out public software. My personal view, not to be 
forced on anyone else, is that fitting genewise mixed-effect models is not 
enough when the number of microarrays is small.

>5. Using randomizedBlock
>In a simple set up of control and treatment with dye-swaps, I have done some
>numerical comparisons of the outcome of a t-test on the mean of the technical
>replicates with lme and with randomizedBlock. [The function is attached]. The
>outcome of the t-test of the means of replicates and randomizedBlock, in
>terms of the t-statistic, tends to be the same (if we "positivize" the dye
>swaps). The only difference, then, lies in the df we then use to put a
>p-value on the statistic. But I don't see how we can use the dfs from
>randomizedBlock: they seem way too large. Where am I getting lost?

The function randomizedBlock() does REML for the variance components plus 
weighted least squares for the fixed effect coefficients for general 
unbalanced mixed models with only two variance components (including the 
usual error variance). It isn't restricted to RCB designs which of course 
are much simpler. As far as I know, there is no theory establishing what is 
the right d.f. for testing contrasts in such models, although I have my own 
ideas and lme() does something ad hoc and conservative. What 
randomizedBlock() returns is simply the df from the weighted least squares, 
i.e., the df take into account estimation of the fixed effects only and not 
estimation of the variance components.


>Ramón Díaz-Uriarte
>Bioinformatics Unit
>Centro Nacional de Investigaciones Oncológicas (CNIO)
>(Spanish National Cancer Center)
>Melchor Fernández Almagro, 3
>28029 Madrid (Spain)
>Fax: +-34-91-224-6972
>Phone: +-34-91-224-6900
>PGP KeyID: 0xE89B3462

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