R help - Mar 2009 - MANOVA

Hi All,

I have questions about MANOVA which I am still not sure if appropriately I
should use it.

For example I have a data set like this:

BloodPressure (BP)  Weight   Height
120        115    165
125        145    198
156        99      176

I know that BloodPressure is correlated with both Weight and Height, however
colinearity exists between Weight and Height. When I use BP = Weight + Height as
the model, one is got to be insignificant. I was trying to use a BP + Weight =
Height model, but not sure how to use it.

Should I use MANOVA? or I just have to do two equations as BP = Weight &
Weight = Height

Any suggestions and answers are greatly appreciated!

Ding

	[[alternative HTML version deleted]]

You only have one response variable, so MANOVA is not appropriate. One
option would be to compare BP ~ Weight + Height with BP ~ 1. That would
give you a joint test of weight and height together. Since they are
collinear, that should tell you the overall effect of "size". There
are
other options, most of which involve discarding some of the data. Frank
Harrell's book is a font of wisdom on this sort of thing.

Harrell, F. E., Jr. (2001). Regression Modeling Strategies. Springer.

Simon.

On Thu, 2009-03-12 at 00:20 -0600, Ding Xiao wrote:
> Hi All,
> 
> I have questions about MANOVA which I am still not sure if appropriately I
should use it.
> 
> For example I have a data set like this:
> 
> BloodPressure (BP)  Weight   Height
> 120        115    165
> 125        145    198
> 156        99      176
> 
> I know that BloodPressure is correlated with both Weight and Height,
however colinearity exists between Weight and Height. When I use BP = Weight +
Height as the model, one is got to be insignificant. I was trying to use a BP +
Weight = Height model, but not sure how to use it.
> 
> Should I use MANOVA? or I just have to do two equations as BP = Weight
& Weight = Height
> 
> Any suggestions and answers are greatly appreciated!
> 
> Ding
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
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> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
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-- 
Simon Blomberg, BSc (Hons), PhD, MAppStat. 
Lecturer and Consultant Statistician 
School of Biological Sciences
The University of Queensland 
St. Lucia Queensland 4072 
Australia
Room 320 Goddard Building (8)
T: +61 7 3365 2506
http://www.uq.edu.au/~uqsblomb
email: S.Blomberg1_at_uq.edu.au

Policies:
1.  I will NOT analyse your data for you.
2.  Your deadline is your problem.

The combination of some data and an aching desire for 
an answer does not ensure that a reasonable answer can 
be extracted from a given body of data. - John Tukey.

No, MANOVA is for Multivariate analysis of variance which is used if there
are multiple responses as well as variables but you just have one response
which is blood pressure. You should just have
model <- lm(BP ~Weight+Height)
anova(model)
If Weight is related to Height only one should be significant so you can
drop it from the model and then use anova again to compare the two models. 
The collinearity between Weight and Height is really a separate question and
shouldn't be a problem when fitting a model for BP.





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