How to find which variables are related to a given variable? Can one use PCA? I have a table which contains about 80 variables. I will give one variable as input and as output I want to have all the variables that are related to the input variable.
Example: I have hospital data which contains variables such as patient name, blood pressure, sugar, heart rate, ECG, "heart patient" (binary yes/no variable), etc. If I give "heart patient" as input, then as output I would get ECG, heart rate, blood pressure, etc.
So I need to find variables which are related to the "heart patient". And the rest will be ignored.
I am using principle component analysis (PCA) to solve this. The problem is that PCA gives me the number of variables which best represent the data set, not the variables that are related to a given variable.
Please suggest me some idea or keywords that help me to proceed. Also, could you give me some reading which would improve my PCA understanding. I have already read Making sense of principal component analysis, eigenvectors & eigenvalues.
 A: I don't immediately see how PCA would be useful here, since PCA is more of an approach for characterizing an entire dataset in one go than a way of characterizing all the variables in terms of a single specially chosen variable.
I think the simplest approach to this problem is to, given the input variable, calculate its correlation (Kendall's τ is a nice general-purpose notion of correlation) with every other variable. Sort these correlations by absolute value to make a list of the variables in order of how closely related they are to the input variable. Choose a threshold absolute correlation, or take the n most closely related, and you have a subset of the variables that are more related to the input variable than the other variables are.
A: What PCA does is "squeeze" the variance of the original variables into a smaller set of variables (the principal components). The problem you describe requires a Feature Selection algorithm or a simple correlation analysis between the input variables and your chosen output.  If you insist (or you're required) to do the task using PCA, then search the literature for "feature selection using pca". However, I suggest you don't go in the latter direction if you're just starting to use PCA.
