Crashing in the Critical Path Method assumes that the cost of crashing an activity is linearly proportional

Question:

Crashing in the Critical Path Method assumes that the cost of crashing an activity is linearly proportional to the amount of time the activity is crashed; that is, the rate of cost increase is constant (see Exhibit 18.12). Is this a reasonable assumption? Why or why not? How might the concepts of economies and diseconomies of scale help to address this issue?