Finding sensitivity or cross correlation?

Hi

I am trying to solve the following problem.

Lets say I have a nonlinear equation y = f(x).

y here is a scalar x is a vector containing [a b c d];

a, b, c, d are all time series. Given the fact that I have a huge time series matrix with many x's (x(:,1), x(:,2), etc.), how can I analyse how sensitive y is to a, or b, or, c, or d.

Can this be somehow done via xcorr? Or do I need to employ any other sensitivity analysis tool in MATLAB?