Re: [R] improvement of Ancova analysis

From: Stephan Kolassa <Stephan.Kolassa_at_gmx.de>
Date: Sun, 04 May 2008 09:05:26 +0200

Hi Tobias,

If you want to do inferential statistics with groups differing systematically on the covariate, you will need to be extra careful in your interpretation. See, e.g., Miller, G. A. & Chapman, J. P. Misunderstanding Analysis of Covariance, Journal of Abnormal Psychology, 2001, 110, 40-48, and a lot of other similar things.

That said, with your wide variation in pes you may want to consider restricted cubic splines ("natural splines") in Frank Harrell's Hmisc and Design packages. At least, it would be interesting to test whether the influence of pes really is linear, which can be done easily with splines. See also Harrell, F. E. Regression Modeling Strategies, Springer, 2001.

Good luck with your small furry creatures! Stephan

Tobias Erik Reiners schrieb:
> Dear Helpers,
>
> I just started working with R and I'm a bit overloaded with information.
>
> My data is from marsupials reindroduced in a area. I have weight(wt),
> hind foot
> lenghts(pes) as continues variables and origin and gender as categorial.
> condition is just the residuals i took from the model.
>

>> names(dat1)

> [1] "wt" "pes" "origin" "gender" "condition"
>
> my model after model simplification so far:
> model1<-lm(log(wt)~log(pes)+origin+gender+gender:log(pes))
> -->six intercepts and two slopes
>
> the problem is i have some things I can't include in my analysis:
> 1.Very different sample sizes for each of the treatments
>> tapply(log(wt),origin,length)

> captive site wild
> 119 149 19
> 2.Substantial differences in the range of values taken by the covariate
> (leg length) between treatments
>> tapply(pes,origin,var)

> captive site wild
> 82.43601 71.44442 60.42544
>> tapply(pes,origin,mean)

> captive site wild
> 147.3261 144.8698 148.2895
>
> 4.Outliers
> 5.Poorly behaved residuals
>
> thanks for the answer I am open minded to any different kind of analysis.
>
> Tobi
>
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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 Sun 04 May 2008 - 07:13:37 GMT

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