Refer to the Significance (December 2015) study estimating the number of U.S. deaths attributable to nitrous oxide pollution (NOx) produced from illegal VW vehicles, Exercise 2.30 (p. 61). The researchers varied the values of three different variables- total distance (in miles) driven by these illegal VW vehicles, the amount by which the VW vehicles exceeded the EPA standard for NOx, and the estimated association between NOx emissions and mortality-to obtain their estimates. Each variable was set at three different levels: Distance (6, 17, and 33 billion miles), Excess amount (15, 30, and 40 times higher than the standard), and Association (.00085, .0019, and .0095 deaths per ton).

a. How many different combinations of Distance, Excess, and Association are possible? The researchers computed the estimated number of deaths for each of these combinations, which they called scenarios.

b. Refer to part a.

Suppose the researchers desire to select two of these scenarios for a more detailed analysis. If these scenarios are selected at random, how many possible choices of two are there?