Ho: z is exogeneous , Ha: not Ho
step 1 : run y on x (all x) by instrument variable method --> save residuals ui( IV hat )
step 2: run ui( IV hat ) on z --> save R^2
step3 : compute LM =n*R^2 ~X^2(m-k) [Chi-squared distribution with degree of freedom m-k]
where m is the number of instrument variables