During the NBA playoffs, series lines are widely available. Bettors that have already picked out teams they like and want exposure on a particular side often ask the question “Should I bet the series line or the money line in individual games?” A follow-up question is "What is the equivalent exposure of a series line bet on an individual game basis?"In this article, I will give the reader a way to answer these two questions given inputs provided by them. First, it is important to lay down some foundations before getting to the two main questions. Those foundations include understanding home court advantage in the NBA Playoffs, the zigzag theory, estimating series odds and adjusting series odds as games are played.
NBA Playoff Format and Home Court Advantage
With the exception of the NBA Finals, the home court format in the NBA playoffs is 2-2-1-1-1. The first two games are at the first team’s home court. The next two games are at the second team’s home court. The fifth game and seventh games are at the first team’s home court and the sixth game is at the second team’s home court.
Home court advantage is significant in the NBA, so having the home court advantage in a series can be significant. Historically, when the two teams are closely matched, the home court advantage has been greater. When one team is much better than the other team, the home court advantage is not as big. If two teams are evenly matched, the home court advantage may be 5-points. The first team would be 5-point favorites at home while the second team would also be 5-point favorites at home. When one team is much better than the other team, the home court advantage is smaller. For example, the Celtics were 15-point favorites at home against the Hawks in games 1 and 2 in their series, but the Celtics were 8-point and 9-point favorites in Atlanta in games 3 and 4 respectively. This reflects only a 3- or 3.5-point home court advantage. In the Dallas-New Orleans series, the Hornets were 4.5 and 3.5-point favorites at home in games 1 and 2 while the Mavericks were 6- and 4.5-point favorites at home in games 3 and 4. This reflects a home-court advantage of about 4.6 points.
TheZigzag Theory
I wrote about the Zigzag theory in the NBA playoffs in Weighing the Odds in Sports Betting. Rather than try to convey the same thoughts in different words, here it is as it appears in the book:
“The simplest form of the zigzag theory in the NBA playoffs is: The team that lost a game is likely to cover the spread in the next game. For example, if the Lakers lost to the Suns in game 1 of the playoff series, the zigzag theory suggest betting on the Lakers to cover in game 2.
From a database of NBA playoff games from 2000 to 2006, the zigzag theory’s record was 231-211 for a winning percentage of 52.3%, basically break-even for -110 lines.
A refinement to the zigzag theory is to add in the qualifications that the losing team in the previous game did not cover the point spread. With that refinement, the zigzag theory’s record was 190-163 for a winning percentage of 53.8%. Data miners can get better winning percentages with more refinements, but testing them runs into the problem of small sample sizes.
I do not recommend you bet the zigzag theory based on this discussion alone. Line makers learn from history and sportsbooks adjust to action. Because the zigzag theory is well known, line makers probably have a good handle on its historical performance. Lines in the NBA playoffs may already have the zigzag factor embedded in them. Blindly betting the zigzag theory may be negative EV in future games. “
Excel Spreadsheet to Estimate Series odds
At SharpSportsBetting.com, there is a free spreadsheet with my formulas for calculating series odds given user inputs. Go to SharpSportsBetting.com to download the file. It is on the left margin, under "Links", “Prop Tools”. You will be able to save a file titled SSB_Prop_Analysis.xls. In the last tab is the NBA Playoff Series estimate. The user has to put in the value of a point as well as the expected point spread for each game. It is a static spreadsheet, meaning that the spreadsheet assumes the point spreads stay constant regardless of previous results in the series; it does not assume a zigzag pattern. This is relatively simple spreadsheet, and just like any formula with user inputs, it is only as good as the inputs.
Series line versus money lines in individual game
A choice that bettors often face once they know they like a specific team to win a playoff series is whether to bet the team in the series line or the money lines of individual games. The key to this choice is forecasting lines in future games. Usually before a series starts, bettors will know the lines for game 1 as well as the series. There is no need to guess the line for game 1, but there is a need to estimate the lines in the other games. The spreadsheet mentioned in the previous section answers that question assuming there is no zigzag theory. Users can always make adjustments assuming a zigzag theory by putting in multiple entries to see series odds. At the beginning of the series, these adjustments will not be much different from the assumption of no zigzag. However, when the series gets down to fewer games, the impact is greater.
If a series is tied 2-2 going into the game 5, the line for the sixth game will usually be slanted towards the team that lost game 5. For example, if Team A lost game 5, Team A may be a 7-point favorite in game 6, whereas if Team A won game 5, they may only be a 5-point favorite in game 6. As games are played and the series progresses, many sportsbooks will adjust odds on series lines. Therefore, understanding the zigzag theory and using the spreadsheet intelligently can be helpful in finding positive EV bets.
Betting the series line has the advantage of making only one bet. Theoretically, bettors pay the bid-ask spread (although sometimes lines can have inverted markets where the bettor can arbitrage the same line at two different sportsbooks). There is no guarantee that the money line in future games will have tight spreads. If the line is as expected, but the spreads are too wide, the bettor may find himself passing on a game in the series because he is paying too much vigorish on that particular game. With the series line, the bettor does not have to worry about finding tight lines on many different games.
In my experience, betting the money line in individual games is a better option on big underdogs (assuming you want exposure on the big underdog). When a big underdog wins a game or two, the betting market seem to lean more heavily towards the favorite than in other situations. The risk in waiting for the money line in individual games is if the favorite has an injury to a star player or if there is an otherwise clear-cut change in the teams (the 2003-2004 Finals between the Lakers and the Pistons come to mind). Then the bettor cannot get the same value on the underdog, at least compared to what he had expected before the series began.
What is the equivalent amount to bet?
Say you have decided to bet the money line in individual game but you would like to have the same exposure as the series line. The question that arises is what is the equivalent amount to bet to have the same exposure? To answer that question, first assume there is no zigzag theory (we will adjust for that later). You estimate that the home team in each game has a 64% chance of winning. For Team A, that means an estimate of 64% for games 1, 2, 5 and 7 and 36% for games 3, 4 and 6. If you want to follow along using the spreadsheet mentioned above, the inputs I used are 3.5% for the value of a point and the home team being 4-point favorites in each game.
Based on these projections, the series percentages are 54.6% (-120 in the money line) for Team A and 45.4% (+120 in the money line) for Team B.
In order to find out how much the equivalent bet is on the money line in game 1 compared to a series line, we have to see what happens to the series percentage when Team A wins and when Team A loses. When Team A wins game 1, the series percentages changes to 66.4% for Team A and 33.6% for Team B. When Team A loses game 1, the series percentage changes to 33.6% for Team A and 66.4% for Team B.
Using these assumptions, if you had a series bet on Team A at fair value (which we estimated at -120) for $120 to win $100, then the expected value of your ticket after game 1 is:
Team A wins game 1 = (66.4% x 100) + (33.6% x -120) = +26.08
Team A loses game 1 = (33.6% x 100) + (66.4% x -120) = -46.08
In effect, your series bet on Team A of $120 to win $100 is the same as betting $46.08 to win $26.08 in game 1, which is equivalent to -177. Note that numbers may be slightly off due to rounding error. The difference is that the series bet still has exposure and risk after game 1 because the series is not yet over.
Bettors can repeat this process at different stages in the series. The risk in any individual game is larger when the series is tight. For example, when the series is tied 3-3, the entire series bet hinges on just one game. On the other hand, when one team is up 3-0 then the next game does not have much risk in the series bet because even if the team up 3-0 loses game 4, they still have three games to win the series.
Additionally, when the series is tied 2-2, the following game is much more important in determining the winner of the series than when it is tied 1-1 or 0-0.
What is the equivalent amount to bet? Part 2
In the previous section, we ignored the zigzag theory. In reality, that is a small mistake. If Team A wins game 1, they will probably be a smaller favorite in game 2. Maybe they will go from a 4-point favorite in game 1 to a 3.5-point favorite in game 2. On the other hand, if Team A loses game 1, then they are likely to be a bigger favorite in game 2. Assume Team A will be a 4.5-point favorite in game 2 if they lose game 1 and a 3.5-point favorite in game 2 if they win game 1. Holding the lines in all other games constant, here are the adjusted series percentages after game 1 with and without using the zigzag adjustment.
|
Adjusted series perc. after Game 1 | Mark-to-market Profit/Loss | ||
| Team A | No ZigZag | With ZigZag | No ZigZag | With ZigZag |
| Wins Game 1 | 66.40% | 65.80% | 26.08 | 24.76 |
| Loses game 1 | 33.60% | 34.10% | (46.08) | (44.98) |
The difference is not that big, it is only 0.6% and 0.5%. The mark-to-market numbers also change, as seen in the table, but not too much. Still, there is a difference in the numbers and that can lead to different decisions at times.
Conclusion
The answer to the two questions posed at the beginning of this article depends on user input. If the bettor can forecast future lines well, then the bettor can use the spreadsheet referred to in the article to estimate whether the series line or the money line on individual games presents better value. The bettor can also estimate the amount to bet in the money line of each game to equate to a particular risk level in the series line.
In this article, I have presented some tools and thoughts for bettors. Tools are only useful in the hands of people that understand how to use them well. Make sure to understand the idea and structure of the spreadsheet and the reasons for marking-to-market and you can use the tools to your advantage.