 Here, formula_9 is not exogenous for formula_1 and formula_3, since, given formula_9, the distribution of formula_12 depends not only on formula_1 and formula_3, but also on formula_10 and formula_24.Suppose that a perfect measure of an independent variable is impossible.That is, instead of observing formula_25, what is actually observed is formula_26 where formula_27 is the measurement error or "noise".