I am trying to understand how the effect of "in" predictors are removed from the "out" predictors in the native matlab code stepwisefit.m.
The code is doing the following but unfortunately I fail to understand why.
% Compute separate added-variable coeffs and their standard errors
xr = x - Q*(Q'*x); % remove effect of "in" predictors on "out" predictors
Any explanation as to what this is doing would be really helpful and highly appreciated.