True Performance Models
This page describes how I create my “True” models: True Economy, True Strike Rate, True Balls Per Wicket
The purpose of the models is to re-create standard measures of player performance but also account for the level of difficulty given the match situation. The overall approach is extremely similar to my win probability and expected run models (see here). The differences are what we are trying to predict and it doesn’t consider as many factors
Below is a list of the factors that I considered and why I include / exclude each of them...
Included
Ball number: Obvious. Teams expect to score more runs during the Powerplay than the middle overs and the run rate gradually ramps up again towards the end of an innings
Powerplay flag: This is more of a technical issue. Without this the statistical approach used struggles to produce a sharp distinction between Over 6 and Over 7
Required run rate: Batsmen chasing a low total will take fewer risks. If this factor is not included, then the models would penalise batsmen who adjust their batting down to the required rate. The converse applies to a lesser extent for bowlers
Balls faced: We expect a batsman who has faced 20 balls already to score faster than a batsman who has faced none. This adjustment is vital if we are to compare opening batsmen with middle order batsmen on any sort of level playing field
Venue: Some venues are easier to score at than others. The are two components to assessing venue: One is based on the historic scoring rates at each ground. The other is a slightly softer adjustment that gives extra credit to batsman who score in matches were all other batsmen struggled (and vice versa for bowlers)
Competition: The ratings are calibrated so that the average True Strike Rate (for example) is equal to the average Strike Rate in the Big Bash League. This is an arbitrary decision and could easily be changed to any other league. Players who accumulate stats in lower quality league (e.g. Blast and BPL) will be penalised slightly whilst performances in the IPL are given extra credit
Excluded
Wickets lost: This was a tough one. The number of wickets lost significantly affects the expected run rate. However, from a batting perspective, wickets lost is highly correlated with batting order. For the sake of simplicity in the underlying model, I prefer to treat all batting positions the same and adjust for this as a separate issue. From a bowling perspective, it should average out in the long run