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Points Efficiency Ratio: The Best Stat In Volleyball
Introduction
When you are looking for the best stat in volleyball, what you are really looking for is a performance indicator. A performance indicator in sport normalises performance data into percentages, ratios or per set/ match averages. This is so it creates a level playing field so we understand and compare different levels of performance. Performance indicators should also be valid, i.e. associated with a successful performance or outcome. Success in volleyball can be defined as set outcome (winning or losing a set) and final league points (that determine league position/ ranking).
Points Efficiency Ratio – The Best Stat in Volleyball
I will put forward that the Points Efficiency Ratio (PER) is the best stat, or performance indicator, in volleyball. It is calculated as Points Scored (Kills, Block Kills, Aces) divided by Points Lost (all Errors). The reason why I believe the PER is the best performance indicator in volleyball is because it is highly correlated to success in volleyball and it is easily calculable to any team collecting basic stats.
Points Efficiency Ratio’s Association to a Successful Outcome in Volleyball
I actually tested PER as a valid performance indicator in my Masters dissertation using data collected during the 2012/13 season of Men’s Super 8s League in England (read the abridged version here). It came out as the second best performance indicator, in terms of significance, that is related to set outcome and league points. The level of significance is mind-boggling too. For those who remember, significant results is usually set at p < 0.05 or sometimes at p < 0.01. The significance for PER in set outcome was p = 0.00000000000000000000000009 and for league points was p = 0.0000006. So the chances that the results came about by chance or luck is astronomically low.
Anyway, I decided to do some graph versions to demonstrate the correlation between PER and set outcome and final league points. I used data from my dissertation – the Men’s Super 8s League 2012/13 season. I’ve also used data collected in the Women’s Super 8s League 2017/18 season since it was the season I did the most opposition scouting.
In the above graph, I calculated a team’s PER over the regular season and charted it against the team’s final league points. 44 out of 56 regular season matches was statted. The final league points can be found on this link. You can see that there is a clear correlation between a team’s PER and the team’s final league points. The higher the team’s PER, the more league points, or wins, a team will accrue.
I did the same graph but used data collected in the Women’s Super 8s 2017/18 season. The final league points can be found on this link. 27 out of 56 regular season matches was statted. So although some teams have limited data, you can see that the correlation is clear again. The higher the team’s PER, the more league points a team will accrue.
In the above graph, I looked at the difference of the average PER of a winning teams in a set and the average PER of a losing teams in a set (Average PER Difference) and charted it against how many set points the set was won by (Set Points Difference). For example, if the set score was 25-23, the set score difference is 2. I found that a winning’s team’s PER is generally higher than the losing team’s PER and that the bigger the difference, the bigger the winning margin was.
The Reason Behind the Points Efficiency Ratio’s Validity
Points Efficiency Ratio actually makes sense as a highly valid performance indicator. This is because for a team to be successful, they must win matches and to win a match, they must to win 3 sets and to win a set, they must score 25 points before the opponent does. This is why the ability to score points and concurrently not lose points is critical to success.
Final Thoughts
Points Efficiency Ratio is the first thing I look at when I see where the match was won and lost and also to gauge the level of a team. It has an additional advantage that it is easily calculable to any team collecting even basic stats.
*** Update 19 January 2021 | Read another blog post on why I like the Points Efficiency Ratio on thislink***
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