What are matchup ratings?
Matchup ratings are the stars you see all over the site to represent how good or bad a matchup a player has that week. If a player is facing a 5-star matchup, you should expect him to perform better than his average. Conversely, we expect players to perform worse than they usually do against a 1-star matchup. A 3-star matchup is neutral.
How are matchup ratings calculated?
This is easiest to explain with an example. Suppose we want to calculate Denver’s matchup rating against Quarterbacks. We look at the fantasy points scored by quarterbacks against Denver over the past year and compare their performance against Denver to their own average. For example, Patrick Mahomes scored 23.9 fantasy points against Denver in week 4. That’s a lot of fantasy points, but it’s actually below average for Mahomes – though week 5, Mahomes has averaged 27.2 fantasy points per game. We calculate the z-score of this performance – Mahomes’ fantasy performances have a standard deviation of 8.4, so he scored 0.39 standard deviations below his average of 27.2 – a z-score of -0.39.
Once we’ve found the z-score for each Quarterback Denver has faced, we average them together – in this example, Denver has an average z-score of 0.012. This means we expect quarter backs to perform just 0.012 standard deviations better than their average against Denver. Relatively strong performances by Denver’s other opponents have balanced out Mahomes’ below average week, to make them a nearly neutral matchup. To convert this to a star rating, we consider an average z-score of -0.5 or below to be a 1-star matchup, and an average of 0.5 or greater to be a 5-star matchup. Denver’s average z-score of 0.0.12 translates into Denver being a 3-star matchup for quarterbacks.
Why not just use points against?
The main advantage of our method compared to raw points against totals is that it adjusts for strength of schedule. A team could have a high points allowed to a position because they’ve faced strong players, even if those players did worse than they usually do against that team.
Do you only consider the current season?
We calculate star ratings based on the past 16 weeks of regular season play, excluding week 17. For week 6 of 2018, we use weeks 1-5 of 2018, and weeks 6-16 of 2017. We weight weeks of the current season twice as heavily as the prior season, so the five weeks of the 2018 season in this example make up 48% (10/21) of the calculation. Preseason and Playoff performances aren’t included.
Wouldn’t bench players who score 0 points every week push the z-scores toward 0, underestimating the impact of matchup?
For this reason, we look at position groups rather than individual players. When evaluating how the Seahawks fare against wide receivers, we look at the total points scored against them by Rams wide receivers as a group compared to how many points that group scores in an average week.
How often are matchup ratings updated?
We calculate new ratings every Tuesday morning to incorporate stats from the most recent week.