Regression -- multicolinearity and VIFs I understand that variance inflation factors can be used to detect multicollinearity. 
What is the intuition behind the VIF formulation? What aspect of this formula shows it detects multicollinearity between the predictors?
 A: The VIF formula for predictor variable $k$ is defined as
$$ VIF_k = \frac{1}{1-R^2 _k }$$
where $R^2_k$ is the coefficient of multiple determination of the regression of the variable $k$ on all other predictor variables. If there is linear dependence among predictors the $R^2$ would then be high because this is precisely what it measures, the explained variance. But when $R^2$ is large the denominator will be small and consequenty the $VIF$ will be large. In fact, it will be unbounded for values of $R^2$ quiet close to $1$ - I believe $10$ is the number frequently used to indicate trouble but that is rather arbitrary.
Note that this is a better approach of diagnosing VIF than looking at pairwise correlation coefficients as there might be a linear relationship among more than two variables and this cannot be detected by the correlation coefficient. A better yet measure is the minimum eigenvalue of the predictor covariance matrix which as you know will be very close to zero in case of degeneracies. 
