Economic variables are often measured with error. Does this mean that regression analysis is unreliable? Explain.

$9\mathrm{.}4$ Suppose that a state offered voluntary standardized tests to all its third graders and that the resulting data were used in a study of class size on student performance. Explain how sample selection bias might invalidate the results.

$9\mathrm{.}5$ A researcher estimates the effect on crime rates of spending on police by using city$-$level data. Explain how simultaneous causality might invalidate the results.