The Learning Curve: The most important part of this short section is Figure 6.8.  It should be emphasized that the measurements on the horizontal axes in the two panels of the figure are different.  In Panel (a) it is cumulative output over time – learning by doing is decreasing average cost with additional experience.  In Panel (b), however, the horizontal axis measures output per unit of time or per period.  In this case we cannot observe learning by doing by moving to the right in the diagram, rather this effect appears in the form of a downward shift of the AC.  This is similar to the discussion of shifts vs. movements in Chapter 2 on Supply and Demand.