The following is part four of a six part analysis (parts one, two and three) on certain factors that are perceived to influence a bettor’s decision when wagering on National Football League (NFL) regular season games against the spread (ATS). These studies look at past trends to assess expected future behavior. Consequently, no weight is placed on current rosters, coaching staffs and matchups. In this study, I examine and compare the past performance of favorites and underdogs against the spread.
Conventional wisdom is to root for (and put your money down on) frontrunners. Oddsmakers know America loves the Cowboys. The Steelers. The Packers. The Raiders. But what is conventional wisdom among hardcore football fans. There remain two factions. When it comes to betting against the spread, the world is divided into those whose faith rests with the favorite and those who firmly believe in the competitive spirit of the underdog.
On October 21, 2001, the favored St. Louis Rams strolled into the Meadowlands as a touchdown favorite and pummeled the New York Jets 34-14 behind Trung Canidate’s unprecedented 195 yards rushing. That week, the Rams were the only favorite to win against the spread. Underdogs went 12-1 during this particular Week 6 and those bettors on the underdog side of the fence cashed in.
So how much does America love the Raiders? Enough so that oddsmakers appear to overvalue the silver and black and tempt their fan base when they play versus weaker opponents. Between 2003-12, Oakland has a 10-27-0 ATS record – a paltry ATS winning percentage of 27%. The most successful team against the spread over this period was the New England Patriots with an ATS record of 78-52-4. This is remarkable considering the number of times they played as a favorite since 2003. The Patriots were a favorite in 134 games since 2003 which translates into 84% of the regular season games they played. Coincidentally, the Raiders were favored in the fewest games.
Not only did New England exceed expectations while favored, they also covered 76% of the spreads on the few occasions they were underdogs. This was, by far, the league’s best over the past ten regular seasons. The next best team was Pittsburgh with an ATS winning percentage of 59%. Over the past ten seasons, two of the worst underdogs against the spread were respective division rivals of the Patriots and Steelers – the Jets and Ravens only covered 44% of their spreads during this period.
I began my analysis by researching ATS results all the way back to 1978 before narrowing my focus to the most recent ten seasons. The graph below shows just how dominant underdogs have been over the past 35 years against the spread.
The period from 2003 to 2007 is the only five-year interval that favorites outperformed underdogs against the spread. Unfortunately, I have focused the bulk of my analysis on the past ten regular seasons and underdogs performed as poorly over that ten year stretch as any other ten year period since 1978.
I subsequently looked at the previous five regular seasons to identify whether the ATS results were indeed trending back towards underdogs.
As you can see, not only do underdogs maintain an annual gap between them and favorites, but the gap is widening.
Now that I have reviewed how favorites and underdogs track season-by-season, I thought it would be helpful to understand how they perform against the spread each week. As expected, oddsmakers appear to struggle at the beginning of the season in their spread evaluations. By midseason (about Week 8), favorites and underdogs track and chase each other as they overlap in waves about every three weeks.
Of any weeks in the graph above, Weeks 3, 4 and 5 appear to provide a historical advantage for underdogs against the spread. Another way to look at the annual progression is to split the ATS results up by month.
The first quarter of the season, represented above by August/September, is the largest periodic advantage for either favorites or underdogs throughout the whole season. However, underdogs only successfully beat the spread in 50.4% of games – a figure that might not be significant enough to apply as a general rule.
I shifted my analysis next to how favorites and underdogs perform against the spread within their own division, within their own conference and outside their conference. The graph below shows how favorites perform against the spread in each of these scenarios. With the exception of a minor impact due to pushes, the underdog graph is the reverse of the favorite graph and, therefore, is not included.
For much of the past ten seasons, there was little to discern from the graph above. In-conference ATS records headed in one direction and out-of-conference records headed in the other. The following season would then be vice versa. A trend did begin to develop in 2009 where favorites began to excel against the spread in non-conference matchups. However, in 2012, favorites slipped below 45% in non-conference games and this trend is now in question.
The following graph considers the size of the spread. My goal in this analysis was to assess whether evenly matched teams perform against the spread any differently than teams projected to be a touchdown apart, for example.
In this graph, I used underdogs to illustrate the effect. In 2005 and 2008, underdogs struggled with poor ATS records in games in which the spread was a field goal or less. Other than that, there appears to be no pattern to the success of underdogs based on the size of the spread. Or do we need to look closer?
The graph above shows the ATS winning percentage of underdogs in games in which the spread was at or above a minimum spread. For example, underdogs have an ATS winning percentage of 56% in games with at least a 10 point spread. I found the cliff at a minimum spread of 12 points to be fascinating. Also, the slope of the line between a minimum spread of nine and 10 points is important to note. This indicates to me that it would be advantageous to use a 10-point minimum as the breakeven point.
Further analysis on double digit point spreads can be found in my 2012 analysis – Vegas ‘Las’ Leader – Past Performance of NFL Double-digit Point Spread Favorites.
What about the favorite who must go on the road? Can any meaningful trends be inferred from looking at the results of home underdogs or road favorites? The following graph attempts to do just that.
A common presumption is to stay away from road favorites when wagering with spreads; however, the results above are not conclusive enough to lean one way or another. In fact, over the past ten regular seasons, home dogs have won 47.6% of games against the spread compared to 50.1% for road favorites.
What about prime time? What about Monday nights? Favorites are used to the pressure of big games and the lights of Monday Night Football are no exception.
Like I expected, favorites seem to thrive in prime-time (or on Thanksgiving). Favorites have performed significantly better against the spread on both Mondays and Thursdays.
The following graph is a heat chart. To read this chart, note the description along the left-hand side (or y-axis). This will be the focus of the heat chart. In the example below, the focus is “Favorite Team Winning Percentage”. I have assigned each favorite’s straight-up winning percentage into groups and compared it to similar buckets for each underdog. Green means greater success against the spread for the favorite. And red means less success. The shades of red and green dull the closer the results are to 50% (see scale below).
This chart is grouped by season-to-date straight-up winning percentages. Consequently, I have excluded games played during the first four weeks of the regular season since a team’s early season straight-up winning percentage is not yet statistically reliable.
For example, referring to the chart below, when favorites with a straight-up winning percentage between .250-.349 play visiting teams with a straight-up winning percentage between .450-.549, they have been victorious against the spread in 62% of matchups since 2003. Squares were left white if the sample size was under ten games.
Note how the colored squares in the heat chart skew towards the bottom left of the chart. This is because you rarely have a top team face off against a poor team as an underdog.
There was enough green at the top of this chart to peak my interest. Favorites appear to have been successful against the spread over the past ten seasons if their straight-up winning percentage was less than .350 against underdogs with an equal or better record. On the other hand, favorites display lackluster ATS performance directly below the line of green squares at the top. I suggest that underdogs with a straight-up winning percentage between .350-.649 have been successful against the spread versus teams with a record within the same range.
LOOKING FOR MORE
You can access the other parts of this series below if you are looking for further analysis on the historical factors that may influence your decisions when wagering on regular season NFL games against the spread.
Part 1: Team by Team Comparison Over the Past 5 Seasons
Part 2: Predicting Results Based on a Team’s Recent Performance
Part 3: Comparing Results Between Home Teams and Visitors
Part 5: Impact From a Team not Playing Every Sunday
Part 6: A Look at Games Played Inside and Outside the Division
THE GOLDEN RULES
I have also come up with The Golden Rules of Wagering on NFL Games Against The Spread. This summarizes the six part series and provides you with unbiased rules to follow when making your picks. These rules, if followed without added judgment, would have performed 1,409-1,080-71 against the spread between 2003 and 2012.
In summary, I have extracted the following rules from my analysis of how favorites and underdogs perform against the spread:
- Bet on underdogs in September
- Bet on underdogs when the point spread is at or above 10
- Bet on favorites on Mondays and Thursdays
- Bet on favorites with less than a .350 straight-up winning percentage against a team with an equal or better record. Note that this only applies following Week 4.
- Bet on underdogs with a straight-up winning percentage between .350-.649 against a team with a record within the same range. Note that this only applies following Week 4.
Bob Sullivan writes periodically for SportingCharts.com and can be followed on Twitter at @mrbobsullivan.
The data on NFL regular season spreads was accessed via Sunshine Forecast whose current source is scoresandodds.com. Spreads are collected by Sunshine Forecast as close as practicable to game time.