Patrick D. (SnarkSD)
January 10 2013 04:15PM
This stastistical study of the percentages (aka: PDO) came to us from Shark Fan and writer SnarkSD. It is an in-depth, thorough investigation, so be sure to settle in for a long, but interesting, read. We will try to answer any questions in the comments for anyone not well versed in statistical theory.
Objective: Define the standard deviation in all strength non-shootout, and even-strength excluding empty net PDO (the sum of shooting percentage and save percentage). Separating the variance into that accounted for by chance, and that accounted for by talent. Investigate variables that may influence PDO. Calculate Points-Per-Game (aka, Expected Points, or EP) for a league average team, given a non-one PDO.
Methods: Data from the 2005-2006 through 2011-2012 season was extracted from NHL.com game-logs and imported into excel and STATA to generate linear and logistic models for all strength data. Data was imported from timeonice.com for even strength excluding empty net data. A program in excel was created to randomly select games, generating thousands of iterations for each regression model, with the average of these models shown below. To generate chance data, the normal approximation interval for binomial proportion confidence intervals was used.
Results: The standard deviations, correlations, and standard error of the model over different samples of games was generated and depicted in table 2. Eg. 1 SD at 30 games for all teams (N=210) is 0.0174, with 1 SD for chance being 0.0140. Figure 2 demonstrates Table 2 over a set of 82 games. Teams that fall outside the red line represent ±3 SD from chance, and are therefore highly likely to be suffering from a low or high PDO from poor or exceptional performance, respectively. League average points-per-game (EP) for a single game is shown in Figure 3. Figure 4 demonstrates EP for multiple game samples.
Conclusion: PDO has boundaries of normal variance, both in observed variance, and variance strictly by chance. With the data below one can measure the expected performance in points-per-game for a given PDO. A deviation from the expected results (ie. points-per-game) indicates a deviation in performance outside of PDO, likely the result of shootout and OT performance if not corrected for, followed by factors that influence PDO intra-game, shot%, home/road, and competition.