In Chapter 5, Section 4 of the textbook, you will return again to the relevant range. Along with the assumption of linearity, the relevant range must be considered when estimating costs using the methods described in this unit. When costs are estimated for a specific level of activity, the assumption is that the activity level is within the relevant range. Costs are estimated assuming that they are linear. Both assumptions are reasonable as long as the relevant range is clearly identified, and the linearity assumption does not significantly distort the resulting cost estimate.