2b. Zimmerman (2014) also includes the following graph (see below). Explain what it shows and why you think they included it. Four years Five years Six years Degree receipt Degree receipt Degree receipt -2 -1 GPA distance GPA distance GPA disiance FIG. 6.-SUS BA receipt by years elapsed since high school. Lines are fitted val- ues based on the main specification. Dots, shown every .05 grade points, are roll- ing averages of values within .05 grade points on either side that have the same value of the threshold-crossing dummy