From: Ben Bolker <bolker_at_ufl.edu>

Date: Thu, 29 May 2008 13:03:56 +0000 (UTC)

R-help_at_r-project.org mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Thu 29 May 2008 - 13:44:36 GMT

Date: Thu, 29 May 2008 13:03:56 +0000 (UTC)

<eesteves <at> ualg.pt> writes:

*>
*

> Dear All,

*> I'me having (much) trouble understanding why it happened and answering
**> a referee's comment to part of a submitted manuscript. I've tried to
**> google for help but... I'm really confident that although this is a
**> R-Help list someone can help me!
**>
**> I used R to do an ANCOVA w/ RNA/DNA as the dep var, sl as the indep
**> var and gut (a factor w/ levels: prey and empty) as the covariate:
**>
**> > RNADNA.sl.gut<-lm(sqrt(RNADNA)~gut*sl,subset=gut!="Yolk-sac",data=cond)
**> > summary(RNADNA.sl.gut)
**>
**> The results from this are:
**>
**> (...)
**> Coefficients:
**> Estimate Std. Error t value Pr(>|t|)
**> (Intercept) 0.856266 0.052252 16.387 < 2e-16 ***
**> gutPrey -0.009568 0.092170 -0.104 0.917
**> sl 0.030575 0.004648 6.578 6.35e-11 ***
**> gutPrey:sl 0.002285 0.007313 0.313 0.755
**> ---
**> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
**>
**> Residual standard error: 0.3312 on 1692 degrees of freedom
**> Multiple R-Squared: 0.05847, Adjusted R-squared: 0.0568
**> F-statistic: 35.02 on 3 and 1692 DF, p-value: < 2.2e-16
**>
**> (...)
**>
**> The question raised by referee is related to the "incompatibility" of
**> the low r2 (0.057) and the high significance (p<<0.0001) of the model.
**> I've interpreted/used this result in the following way: although
**> there's a significant relationship between RNA/DNA and sl, it's very
**> weak; besides, no gut effect on the relationship as been found!
**>
**> Sorry for the off-topic question but...
**>
**> Sincerely, Eduardo Esteves
**>
*

With 1696 data points, a relatively low r^2 can indeed
give a high degree of statistical significance. It's up to
you to convince the reviewers that an increase of 0.03 in
sqrt(RNA/DNA) per unit of sl (whatever

that is) is indeed *biologically* significant and worth
discussing ... but the observed pattern (or one more
extreme, in either direction) is certainly unlikely
by chance if there were no effect of sl on sqrt(RNA/DNA).
(Is sl "standard length" by chance? Is this a size correction?)

Ben Bolker

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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Thu 29 May 2008 - 13:44:36 GMT

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