In Attempting to Estimate the Causal Effect of Employee Hours

Multiple Choice

In attempting to estimate the causal effect of employee hours worked on total units produced, you include the age of the production equipment as a control variable. Which of the following conditions would not be a good reason to include this control variable?

A) Older production equipment often requires more employees to work on them.
B) Older production equipment often is less productive (i.e., produce fewer units on average) .
C) The age distribution of the plants is highly non-normal.
D) The semi-partial correlation of total units produced on age of the machine holding number of employees fixed is positive and statistically significant.