[BioC] Help on factorial experiment analysis using limma

Gordon K Smyth smyth at wehi.EDU.AU
Sun Sep 14 11:36:24 MEST 2003

> Dear Gordon,
> Thanks a lot for your prompt reply. I have more questions.
>> This is a saturated direct design for a two-way factorial experiment.
>> Good.
> Is it still ok to use limma to analyze the data if one or both diagonal
> experiment arrow is missing? I just realize I have another experiment
> that I want to discard the diagonal experiments for some reasons. It
> seems I can still run my analysis on the data without the diagonals by
> limma, but I have no clue whether this is a legitimate analysis or not.

Yes, you can still run your analysis in limma with no problem, except of
course that with less data you will be able to make comparisons with
slightly less precision.

>> This is a good question. For the purposes of the limma analysis, I
>> think I would treat the technical reps as ordinary reps, i.e., treat
>> the experiment
>> has having 12 independent arrays. This will have the consequence that
>> the standard errors from the analysis will be slightly
>> under-estimated, i.e., the significance results will be slightly
>> over-stated, but the ranking of your genes in terms of evidence for
>> differential expression will be close to optimal.
> This reminds me a very general question with regard to replication I
> have had for a while. What is the proper way to analyze the replicated
> data if there are both biological replication and technical replication
> in the raw data? Consider an example in which there are a hundred
> samples from different cancer patients and the microarray experiment for
> each sample is repeated three times. I heard some people would treat
> both biological and technical replicates equally in this case. But isn't
> it true that the technical replicates would have smaller variance and
> are somehow related with each other and should be treated differently?

Yes.  An explicit treatment of technical replicates will be a future
version of limma.

>> More good questions. The design matrix that you've written above
>> corresponds to a classical interaction parametrization. Here the
>> column 'ab' corresponds to extra effect that 'a' has in the presence
>> of 'b'. The effect of 'a' by itself (a0-00) is represented by the
>> coefficient 'a' and the effect of 'a' in the presence of 'b' (ab-0b)
>> is represented by the sum of the coefficients for 'a' and 'ba'. If 'b'
>> is a confounding factor, then you probably want to have the effects
>> for 'a' with and without 'b' in your heatdiagram. You could do this by
>> fit <- lm.series(MA, design)
>> contrast.matrix <- makeContrasts(a,a+ba,levels=design)
>> fit2 <- contrasts.fit(fit, contrast.matrix)
>> eb2 <- ebayes(fit2)
>> heatdiagram(stat=eb2$t,coef=fit2$coef)
>> This would show whether genes which respond to factor 'a' still
>> respond to 'a' in the presence of 'b', and whether in the same
>> direction. Note that makeContrasts() is available only in the
>> development version of limma.
> Could you give me further assistance on understanding the biological
> meaning of the heatdiagram?

I can explain the statistical meaning.  The biological interpretation is
really more in the domain of biologists such as yourself.

> I know red means the gene is upregulated and
> green means the gene is downregulated. Does white color mean the
> statistical test is not significant for that gene?


> In my particular
> example, what does it mean when the gene i is
> i. red for a and green/less red for a+ba? Does it mean b is suppressing
> the effect of a on this gene?

red for a and green for a+ba (which remember means the comparison ab-0b)
means that a upregulates the gene but b *reverses* the effect of a.

red for a and less red for a+ab could mean that b is reducing the effect
of a.  The interaction term ab tells you whether this reduction is
actually significant.

> ii. red for a and white for a+ba? Does it mean b has no effect on this
> gene while a is upregulating it?

No.  a upregulates the gene but b blocks the effect of a.

> iii. red for a and more red for a+ba? Does it mean a and b are both
> upregulating gene i?

Yes.  a upregulates the gene and even more so in the presence of b.

> (I won't consider the green situations for a because they are
> essentially the same)
> This question sounds a little bit stupid, but I believe a step-by-step
> guide is actually very helpful for biologist like me to understand the
> result. It would be nice to have more tutorials on how to use
> heatdiagram, make contrast, venndiagram etc to interpret the biological
> meaning of analysis result in the manual of your future version of
> limma.

If you or someone else could provide me with an interesting data set on
which the illustrate the methods, then I could include it in the User's
Guide.  At present I do not have permission to make public any data sets
on which I have used these methods.


> Thanks very much!
> Best regards,
> Fai

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