R help - Aug 2006 - Is the PC1 in PCA always a "size effect"?

Hi

Im a beginner to R or statistics in general.

I am comparing our products(6 in all) and competitive products(2). These
products have a similar chemical formulation, but slightly different to each
other. I chose the quantity of each chemicals as the variables and did a PCA to
extract the characteristics of the products. The result was a PC1 which could be
interpreted as the difference between us and the competitive. In my previous
experience, PCA always gave me a PC1 that is a linear combination of the same
sign, which is called more or less a "size effect" as is explained in
statistics textbooks. Is there anything wrong in my PCA or in my interpretation
of the result? Does the same thing often happen in other field too?

Thanks in advance!
-- 
Yukihiro Ishii <yukiasais at ybb.ne.jp>

Hi

Im a beginner to R or statistics in general.

I am comparing our products(6 in all) and competitive products(2). These
products have a similar chemical formulation, but slightly different to each
other. I chose the quantity of each chemicals as the variables and did a PCA to
extract the characteristics of the products. The result was a PC1 which could be
interpreted as the difference between us and the competitive. In my previous
experience, PCA always gave me a PC1 that is a linear combination of the same
sign, which is called more or less a "size effect" as is explained in
statistics textbooks. Is there anything wrong in my PCA or in my interpretation
of the result? Does the same thing often happen in other field too?

Thanks in advance!

Asako Ishii<i_shi_i2005 at yahoo.co.jp>