Selecting input variables

Dear All,
When we are selecting input variables, should there be correlation between the variables?If so how to select them?
I did a parametric analysis using PPA price, SM, number of field subsections, and TES (hrs) for my thesis on CSP. my supervisor asked how do you select these variables as the inputs? what are the correlation between them? what are the statistics behind selecting of inputs? are they have linear relationship or another relationship? How do I answer those questions? please help me.
Other thin is when I click on some links on NREL web site and some google links (of search results of SAM tutorials etc) they are not working. error massages come as "This page isn’t working" or "URL does not have a redirection page ...". Can you help me on these issues?

Dear Mr.paul,
Do you have any guideline or tutorial on this matter. It should include with characteristics of parameters like independent variable, dependent variables, correlated variables etc, and also limitations of parametric analysis which is carried out in SAM.

Dear paul,
I went through the webinar and the technical manual for parabolic trough physical model. But couldn't get clear answer.
My problems are
1. Why do we do a parametric analysis? why don't we do a non-parametric analysis?
2. When we do a parametric analysis what are the assumptions and limitations? here I want statistical answer. that means, that should address correlations between inputs,sample sizes, hypothesis based on, Statistical power, different spread of each group,skewed and non-normal distributions and p value etc...

Dear paul,
I went through the webinar and the technical manual for parabolic trough physical model. But couldn't get clear answer.
My problems are
1. Why do we do a parametric analysis? why don't we do a non-parametric analysis?
2. When we do a parametric analysis what are the assumptions and limitations? here I want statistical answer. that means, that should address correlations between inputs,sample sizes, hypothesis based on, Statistical power, different spread of each group,skewed and non-normal distributions and p value etc...