If you are interested in defining, as simply as possible, what makes a "good friend," you might list all of the things you value in a friend, and then use a statistical procedure based on correlations to narrow all of these down into just a few basic categories or dimensions. This type of analysis is called

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Factor analysis is the statistical method used to identify underlying variables, or factors, that explain the pattern of correlations within a set of observed variables. In this context, it would help in narrowing down a large list of valued traits in a friend to a few fundamental categories.

If You Are Interested in Defining, as Simply as Possible