The prevailing view is that Vegas is an example of an efficient market. If there were obvious trends that oddsmakers ignored, it would be easy for people to make money gambling on football, and we know that’s not the case. But I thought it would be interesting to investigate some claims I’ve heard over the years, so I’m introducing the Efficient Vegas tag to Football Perspective.

One theory I’ve heard is that when good teams play bad teams, the smart money is to bet on the bad teams. That’s not because Vegas doesn’t know what it was doing, but that oddsmakers know that fans like to bet on good teams when they play bad ones. But is this true? Here is how I decided to test that question.

From 1990 to 2013, there were 792 games that met the following four criteria:

• the game was played in weeks 7 through 16
• entering the game, one team had a winning percentage of 0.600 or better
• entering the game, the opponent had a winning percentage of .400 or lower
• entering the game, the two teams had a difference in winning percentage of at least 0.250

There were 792 such games. In those games, the team with the better record went 378-399-15 against the spread (and 580-211-1 straight up, for a 0.733 winning percentage). If you had bet $110 on the underdog in each game — assuming you need to bet $110 to win $100 — you would have won more often than you lost but you would have still walked away down money due to the vigorish. In other words, this does not appear to be a theory one could use to exploit the public’s tendency to back favorites, and is therefore evidence of an efficient market for Vegas. In fact, one could argue that these results represent a more lucrative market than a true 50/50 split, if we believe that more people tend to back the favorite (since Vegas would be winning more than half the time). For what it’s worth, in 2013, the team with the worse record went 15-17 against the spread in these games, making this “zig against the public” strategy an even worse one than normal.

Some trivia notes. In 36 of the games (4.5% of the sample), the team with the better record was actually the underdog, with two teams being as large as seven-point dogs. Those games were in 2012, when Andrew Luck and the 7-4 Colts upset the 4-7 Lions, and in 1991, when the 5-1 Lions lost to the 2-4 49ers, 35-3. In those 36 games, the underdog team with the better record went just 14-22 against the spread.

If you want to make a suggestion for a future topic of investigation on the efficiency of the Vegas line, post it in the comments.