Don't take this entirely too seriously. I saw the chart at Wm. Briggs, Statistician to the Stars! and followed it back to Gradeinflation.com but I did not dig into the data.
The average grades given for each year by a sample of 79 colleges over the past near-century. Data for the 1930s is sparse, being only about 8 colleges. So it may also be that not all schools are present in all years, esp. in the early part of the graph.
Now here is a remarkable thing regarding infantile regression. Take a ruler -- not a king, but a straight-edge -- and place it over the period of the 1940s. You will notice two things:
- There is not a linear trend. The 1950s bent down; there was a sudden shift starting in the mid-1960s, flattening out after the mid-1970s, and then climbing steadily since about the mid-1980s.
- If you extrapolate from the 1940s, the red overall averages after 1985 falls exactly where your straight edge would have put it had the 50s-60s-70s roller coaster never happened! 'Sup that?
- A third thing is that a grade level gap opened up between private colleges and public colleges in the 1960s. Either private schools like Harvard are easier graders than public schools like Ohio State or else the private schools began getting a smarter student body
If pushed, or nudged with a sharp elbow, TOF might be inclined to say that, in addition to random variation and school-to-school variation (the gray dots!) there are two things happening.
- Grade inflation, as marked by the trend in the 1940s and emerging again after the 1980s.
- A perturbation to the process that caused the system to "bounce around" before settling back on the underlying trend.
You can see the same sort of perturbation behavior in other data.
US Birth Rates. The trendline is a sine wave coasting down an decaying exponential curve. But in 1919 there is an icicle, because in 1918 the guys were off in France doing guy things. Then in 1920-21, the rates spiked above the trendline as all the horny soldiers came home before in 1922 returning to just where the sine wave would have taken the rates had there been no perturbation in the first place. A similar, though bigger dip-spike-normal patters holds in the years after Pearl Harbor.
% Women in the labor force. The trendline is an exponential growth curve. The spike representing Rosie the Riveter drops right back to where the curve would have wound up without the spike.