[R] Continous variables with implausible transformation?

[Bert Gunter]

Below. -- Bert

Bert Gunter
Genentech Nonclinical Biostatistics
 
 

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On
Behalf Of Joris Meys
Sent: Thursday, June 03, 2010 5:20 AM
To: zhu yao
Cc: R-help
Subject: Re: [R] Continous variables with implausible transformation?

x <- rnorm(100,10,1)
sqrtx <- sqrt(x)
y <- rbinom(100,1,0.5)

lrm(y~x+sqrtx)

works. What's the problem?
--------------------------------------
But you wrote linear+ square. Don't you mean:
lrm(Y~x+x^2)

--- I believe this is the same as lrm(Y ~ x).
You must protect the x^2 via

lrm(Y ~ x + I(x^2))

------------------------------------------

Cheers

On Thu, Jun 3, 2010 at 6:34 AM, zhu yao <mailzhuyao at gmail.com> wrote:

> Dear r users
>
> I have a question in coding continuous variables in logistic regression.
>
> When "rcs" is used in transforming variables, sometime it gives
implausible
> associations with the outcome although the model x2 is high.
>
> So what's your tips and tricks in coding continuous variables.
>
> P.S. How to code variables as linear+square in the formula such as lrm.
> lrm(y~x+sqrt(x)) can't work.
>
> Many thanks.
>
> Yao Zhu.
>
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>
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-- 
Joris Meys
Statistical Consultant

Ghent University
Faculty of Bioscience Engineering
Department of Applied mathematics, biometrics and process control

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B-9000 Gent

tel : +32 9 264 59 87
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