Figure D.1 Chart. Example of Regression to the Mean
Figure D.1 is a chart showing three different types of data. Years are on the x-axis and the number of crashes is on the y-axis. One line shows hypothetical 1990 to 2010 annual crash frequency at a site. The crash frequency varies up and down from year to year. The second line represents the long-term average crash frequency at the same hypothetical site. The long-term average stabilizes at approximately 14 crashes per year. The third line represents the five-year rolling average number of crashes per year. The first five-year average is from 1990 to 1994 and is plotted in 1994, the second is from 1991 to 1995 and plotted on 1995. As shown, the five-year rolling average more closely approximates the long-term average than the annual crash frequency alone.