One of the goals I set for myself this summer was to learn a bit about D3, a visualization toolkit that can be used to manipulate and display data on the web. Considering that the trees are bare and we’ve already had our first frost here in Wisconsin, you can safely assume that I am behind schedule. Nevertheless, I feel that I’ve finally reached a point where I have something to publish, so here goes.
I wanted to start out with something simple so I elected to go with a basic line chart using data I pulled from Pro-Fooball-Reference.com. This site contains a ton of great information and statistics from the past 90+ years of the National Football League but — for now — I just looked at the final scores of all the games played from 1920 to 2011. My first D3-powered chart is below. It shows the average combined scores of winning and losing teams for each year of the NFL’s existence.
Although this chart looks pretty simple, every element — including titles, subtitles, axes, labels, grids and data lines — has been created manually using the D3 code. The payoff is pretty nice. All of the elements can be reused and you have tremendous control over what is shown onscreen. To demonstrate some of these cababilities, I’ve added interactive overlays that show a few of the major eras in NFL football (derived from work of David Neft and this discussion thread). If you move your mouse over the graph, you will see these different eras highlighted:
Early NFL (1920-1933) – The formation of the American Professional Football Association (APFA) in 1920 marked the official start of what was to become the National Football League. This era was marked by rapid formation (and dissolution) of small town franchises, vast differences in team capabilities and a focus on a relatively low-scoring running game. At this time, the pass was considered more of an emergency option than a reliable standard. The rapid growth in popularity of the NFL during this era culminated with the introduction of a championship game in 1932.
Introduction of the Forward Pass (1933-1945) – The NFL discontinued the use of collegiate football rules in 1933 and began to develop its own set of rules designed around a faster-paced, higher-scoring style of play. These innovations included the legalization of the forward pass from anywhere behind the line of scrimmage — a change that is often called the “Bronko Nagurski Rule” after his controversial touchdown in the 1932 NFL Playoff Game.
Post-War Era (1945-1959) – The end of WWII saw the expansion of the NFL beyond its East Coast and Midwestern roots with the move of the Cleveland Rams to Los Angeles — the first big-league sports franchise on the West Coast. This period also saw the end of racial segregation (enacted in the 30s) and the start of nationally televised games.
Introduction of the AFL (1959-1966) – Professional football’s surge in popularity led to the formation of a rival organization — the American Football League — in 1960. The growth of the flashy AFL was balanced by a more conservative style of play in the NFL. This style was epitomized by coach Vince Lombardi and the Green Bay Packers, who would win five championships in the 1960s. In 1966, the two leagues agreed to merge as of the 1970 season.
Dead Ball Era (1966-1977) – Driven in part by stringent restrictions on the offensive line, this period is marked by low scores and tough defensive play. Teams that thrived in this environment include some of the most famous defenses in modern NFL history: Pittsburgh’s Steel Curtain, Dallas’ Doomsday Defense, Minnesota’s Purple People Eaters and the Rams’ Fearsome Foursome.
Live Ball Era (1978-present) – Frustrated by the decreasing ability of offenses to score points in 70s, the NFL began to add rules and make other changes to the structure of the game in an attempt to boost scoring. The most famous of these initiatives was the so-called “Mel Blount Rule” (introduced in 1978), which severely restricted the defense’s ability to interfere with passing routes. With the subsequent introduction of the West Coast Offense in 1979 — an offense based on precise, short passes — this period became marked by a major focus on the passing game.
Having created this first chart, I decided to build a second chart based on the ratio of average winning scores to average losing scores to see if there were any patterns.
The chart above shows how — after a period of incredibly lopsided victories — the average scoring differential settled in to a very steady pattern by the late 1940s and stayed at that level (roughly 2:1) for the next 30 years. Despite many changes in rules, coaching techniques, technology and other factors, only the pass interference rules of the late 1970s seemed to have any signifcant effect on this ratio, shifting it to just under 1.8:1 for the next 30 years.
While I had the data available, I also decided to look at the differences in average scores between home teams and away teams. The chart below plots this data along with the same overlay I used in the first chart.
A look at the ratio of average home team scores to average away team scores follows:
What’s fascinating about this chart is how quickly a form of parity was acheived among all the NFL teams. By the mid-30s, a measurable home field advantage can be seen at roughly 15%, a rate that has remained essential constant for over 70 years. Factors for this boost could include the psychological support of fans, familiar weather conditions, unique features of local facilities, lack of travel fatigue, referee bias and/or increased levels of motivation in home town players.
Thanks to Charles Martin Reid for his solution to getting D3 and WordPress to play nice.