On its own, completion percentage is a limited metric due to its treatment of every pass attempt as equally difficult. Instead of simply using completion percentage, we estimate the completion probability of each pass and use it to calculate a quarterback’s Expected Completion Percentage (xCOMP%) which can the be compared to their Actual Completion Percentage (COMP%) to get an idea of how accurate that quarterback truly is.
On this page, you can see the cumulative advanced CPOE data from 2009 to 2019. The advanced CPOE model takes into account many more factors (such as wind speed, temperature, etc) than the basic model, but trade-off is that the basic model dates back to 2006, including an additional three seasons in its data.
For a more detailed explanation of CPOE, read about it here.
Expected Completion Percentage (xCOMP%:) The percentage of attempts a league-average quarterback would be expected to complete.
Actual Completion Percentage (COMP%): The percentage of attempts the quarterback actually completed.
Completion Percentage Over Expectation (CPOE): The difference between the quarterback’s actual (COMP%) and expected (xCOMP%) completion percentage.