For each of the following, identify which kind of variation is at work—random or assignable—and explain your reasoning.
a. During the noon hour, there is an electrical power disruption and nuts machine-tightened during the five minutes that emergency power is on have 15% less torque than normal.
b. Although they are all labeled “9–D,” the shoes on a retailer’s shelf are not exactly the same size. Any given shoe is just as likely to be slightly larger than 9–D as it is to be slightly smaller.
c. The number of cars leaving downtown Cleveland during a Friday rush hour is greater than during other days of the week.
d. In a production run of curtain rods, the lengths are normally distributed, with a mean of 24.135 inches and a standard deviation of 0.142 inches.