In their study of the effectiveness of cardiac catheterization, McClellan, McNeil, and Newhouse XXXXXXXXXXused as an instrument the difference in distance to cardiac catheterization and regular...

In their study of the effectiveness of cardiac catheterization, McClellan,
McNeil, and Newhouse (1994) used as an instrument the difference in
distance to cardiac catheterization and regular hospitals. How could you
determine whether this instrument is relevant? How could you determine
whether this instrument is exogenous?