What's Different About Finals?

Finals, by their nature, tend to pit more-evenly matched teams against one another, on average, than do games from the home-and-away season. It seems reasonable, therefore, to hypothesise that margins will tend to be smaller in Finals than in the home-and-away season, but what other changes in scoring behaviour might we expect to see? Will fewer or more goals be kicked per game, fewer or more behinds, will we see a greater or lesser number of points registered, and more or less accurate kicking.

An analysis of games from the home-and-away and Finals seasons for eight non-overlapping eras appears in the table below. It shows that:

  • In every era, as hypothesised, games margins tend to be smaller in Finals than in games from the home-and-away season. Across history the difference is about 2.6 points, though since 1980 it's been only 1.8 points, the average having been bolstered by large values from early in football's history. The difference in more recent history is smaller than I would have expected.
  • In all eras since 1955 the average aggregate score in Finals has been lower than in games from the home-and-away season, especially in the era 2000 to 2014 where the difference has been over 2 goals.
  • Lower aggregate scores in this period have been mostly the result of two factors: fewer goals per game in Finals and lower Conversion rates. Average behinds per game have been lower in two of the eras and higher in the remaining two, but differ by no more than 0.3 behinds per game in either direction in any of the four eras.

It's interesting to speculate what might be the cause of lower Conversion rates in Finals. One hypothesis would be that the more-evenly matched nature of Finals contests results in scoring opportunities that are, on average, more difficult than those in the home-and-away season.

To test this I looked at the correlation between conversion rates in all games since 2006 with:

  • The absolute difference in the Bookmaker-implied victory probabilities of the two teams (calculated using the Overround Equalising methodology)
  • The absolute Bookmaker handicap in the line market for the game
  • The absolute difference in the competing teams' pre-game MARS Ratings

Respectively, the correlations are +0.08, +0.09 and +0.10. These are all positive, which is consistent with the hypothesis, but the effect size they imply is tiny.

It seems more likely, therefore, that the lower conversion rates in Finals are due either to teams playing a more defensive style in these games, leading to opportunities being, on average, more difficult, or that the pressure of the occasion leads to less accurate kicking and a greater willingness of defences to concede behinds.

To separate and quantify those hypotheses, I'd need more and different data.