This is the first of my posts that I’ll call “Quick Slants”. These posts will be short and to the point, sometimes discussing a current event, a hot topic, or just a random topic I’m interested in. In this post, I want to discuss the difference between descriptive and predictive statistics, in preparation for my next post which will compare what wins games in the regular season versus the postseason.
A descriptive stat is one that describes the past. These types of statistics tell you what happened, not what will happen. Wins and losses are a great example; they tell you exactly what happened but they are no guarantee of the future. Other examples: Win Probability Added (a concept that originated in baseball, and implemented by Brian Burke of Advanced NFL Stats), yards gained, turnovers, and tackles.
Predictive stats are better at doing just what the name implies: predicting the future. For the most part, I am going to be more interested in predictive statistics. I’m more interested in how a player will do than how he has performed. Certainly the past can be a good indicator of the future, but it is in determining which statistics hold predictive power and which are the product of luck and randomness that the real value lies. Stats that are more predictive than descriptive are yards per play, points per possession, and Expected Points (another Brian Burke creation).
Now this is an oversimplified view, but I wanted to give a summary. In reality, statistics are some combination of predictive and descriptive. It is also, obviously, extremely important exactly what you are trying to predict. Predicting which team is going to win is certainly going to rely on different numbers than trying to determine how many yards a running back will rush for. The point is that when tackling (pardon my pun) a problem, it is important to define exactly what you are trying to do. Are you trying to describe the past? Or are you trying to predict the future? That is the first step in any process to study a question, and will guide you towards the numbers that will help you best achieve your answer.