How Accurate Are Vegas Lines?

I’ve heard it many times…Vegas knows! But I wondered. How good are the lines?

I was unhappy with what I could find on the internet so I decided to look into it. To start, I defined what I wanted to know. How accurate are Vegas sports betting lines? To be more specific, I wanted to know the average difference between the line Vegas set for a game and the actual outcome. For example, New England was 4.5 point favorites over Philadelphia in Super Bowl LII. Philadelphia won the game 41-33. So in this example Vegas was 12.5 points off on setting the spread (Philadelphia was expected to lose by 4.5 points and won by 8 points).

But is this typical? In Super Bowl LI New England was 3 point favorites over Atlanta and won the game by 6 points. So Vegas was off by only 3 points in this example and pretty accurately predicted the outcome of the game.

In this instance, we need to look at a larger sample of games. For this analysis I choose college basketball for a few reasons.

• College basketball is a mainstream sport which means many people want to place bets on it which is one check on accuracy.
• College basketball has a lot of games, which means larger populations to analyze. There are 256 NFL games a year (32 teams play 16 games) while in college basketball there may be 4,000 games in a season with a spread. Larger data sets allow for different types of analysis.

For purposes of analyzing the data, the metric Absolute Difference from Spread (ADS) was calculated for every game in the data set. For 22,215 college basketball games in the population, the median ADS was 7 and the mean ADS was 8.6. To state this another way, half of college basketball games are within 7 points of the spread and half are greater than 7 points. 

View the data in chart form.

Is this good? We will explore this in a later post.

Data

• The data used for this analysis is 22,215 college basketball games played between November 9, 2012 and January 31, 2018.
• Absolute Difference from Spread (ADS) is the difference between the predicted outcome of the game and the actual outcome of the game measured using spread. For example, if Team A is favored by 15 and wins by 13, the ADS is 2. However, if Team A is favored by 5 and loses by 2 the ADS is 7.
• The team with the largest ADS was Pittsburgh which on January 24, 2017 was 5 point underdogs to Louisville yet lost the game by 55 points. By the ADS measure, this was the most lopsided game in the last five years.