On Jan 24, 2015, at 11:02 PM, Moxley, Jerad wrote:

> Sorry I was not clearer, but I was asking an R programming question not a theory question.

Copying back the original text which appears to have been omitted:

On 25/01/2015 02:01, Moxley, Jerad wrote:

> I’m trying to test what growth functions best fit individual subjects. I’m wanting compare linear, quadratic, cubic etc. Here is the example from the cubic curve.

>

> b3a<-by(c,id,function(x) lm(w~agec+ageq+agecub,data=x))

> I will try to clarify. If I did this analysis with a dataset involving just one subject the summary command on the lm object would give me a significance test on each parameter fit. The question in this cSe, is the cubic parameter significant?

It's not really true that you are "not asking a theory question". You do want inferences. A further problem might be that people who do statistics regularly would not be making statistical inferences based on naive construction of squared and cubed values. I suspect most readers would rather not comment on a procedure that seems particularly prone to invalid results.

--

David.

> When I try to do this for each subject separately in a larger dataset using the by command, I get the parameter estimate but can't find a significance test. I apologize again for my poor explanation originally,

>> On Jan 24, 2015, at 10:04 PM, Jeff Newmiller <

[hidden email]> wrote:

>>

>> R-square is often a poor indicator of whether a model is appropriate or not. While it is possible that there exists a package that implements your algorithm (which you might find using the sos package), I would recommend that you get some advice from an expert on how to approach this subject, and this list is not a good place for studying statistics theory (read the Posting Guide). You might try stats.stackexchange. com.

>> ---------------------------------------------------------------------------

>> Jeff Newmiller The ..... ..... Go Live...

>

David Winsemius

Alameda, CA, USA

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