Over/Under bias in NHL Ice Hockey

Blog, Gambling Blog | | January 14, 2010 at 11:08 am

Many people know about the favourite/longshot bias that occurs in most sports, but little is talked about the overs/unders bias. Is there in fact a bias at all? We shall investigate.

Using data going back to the 2005/06 season, we have calculated the returns should one have simply bet $100 on either of the over or under in the total goals for the match. Which one will come up better? The results are shown below with the first table being the over bets, the second being the unders:

Season#Bets#Wins%Wins$Bet$Profit%ROI
2005/06119757048% $119,700.00 -$7,492.00 -6.3%
2006/07119656347% $119,600.00 -$7,736.00 -6.5%
2007/081250594.548% $125,000.00 -$6,628.00 -5.3%
2008/09126062450% $126,000.00 -$1,575.00 -1.3%
2009/1057527047% $57,500.00 -$3,197.00 -5.6%
Total54782621.548% $547,800.00 -$26,628.00 -4.9%
Season#Bets#Wins%Wins$Bet$Profit%ROI
2005/06119762752% $119,700.00 $2,672.00 2.2%
2006/07119663353% $119,600.00 $3,811.00 3.2%
2007/081250655.552% $125,000.00 $1,291.00 1.0%
2008/09126063650% $126,000.00 -$3,381.00 -2.7%
2009/1057530553% $57,500.00 $808.001.4%
Total54782856.552% $547,800.00 $5,201.00 0.9%

Clenhl2arly, this shows that had one bet $100 on each bet on the over, you would have lost around 4.9% ROI from nearly 5,500 bets. Alternatively, had you bet $100 on the unders, you would have made over $5,000 from nearly 5,500 bets. The bias is definetly there, that unders betting is if anything, the way to go.

Is it possible that this difference is purely due to random variation? Quite possibly, however there is a reason why the bias is there. No-one wants to see a low scoring game, everyone likes plenty of goals and will generally bet with what they want to see. Also, punters will generally remember the high scoring games, the great goals of the year and the best goalscoring players. This in turn biases them towards betting the overs. The fact that in NHL, if the scores are level at end time, we have a playoff, then this I believe, adds more fuel to the fire to say that the bias is there. Because even if the scores are level, there still will be one more goal scored.

For those statistically inclined, the difference is in fact statistically significant. A simple one sample p test shows that the probability of such a result happening due to random variation is less than 1 in a thousand.

So the bias is there, and we have good reason to suggest why it occurs.

Equally interesting, is how the bias works amongst different goal lines. Shown below is the results should one have bet $100 on the overs and unders for different goal lines. Once again, the top table is the over bets, the bottom table is the unders (that makes sense!)

Line#Bets#Wins%Wins$Bet$Profit%ROI
4.5171165% $1,700.00 $342.0020.1%
5719389.554% $71,900.00 $2,630.00 3.7%
5.52513115146% $251,300.00 -$17,590.00 -7.0%
61402670.548% $140,200.00 -$8,088.00 -5.8%
6.574336749% $74,300.00 -$2,168.00 -2.9%
78029.537% $8,000.00 -$2,018.00 -25.2%
7.54375%$400.00$264.0066.0%
54782621.548% $547,800.00 -$26,628.00 -4.9%
Line#Bets#Wins%Wins$Bet$Profit%ROI
4.517635% $1,700.00 -$416.00-24.5%
5719329.546% $71,900.00 -$5,060.00 -7.0%
5.52513136254% $251,300.00 $6,771.00 2.7%
61402731.552% $140,200.00 $3,307.00 2.4%
6.574337651% $74,300.00 -$807.00-1.1%
78050.563% $8,000.00 $1,631.00 20.4%
7.54125%$400.00-$225.00-56.3%
54782856.552% $547,800.00 $5,201.00 0.9%

Most of the bets have a goal line of 5.5 or 6, but the overs for a run line of 5 and below made a nice profit. 736 bets were made for a 4% ROI. I have little doubt, that this is because of an over adjustment by the bookies to a smaller runline due to perhaps several previous low scoring games by the teams involved. Once again, this difference is statistically significant and not due to random variation. Similarly, betting unders with run lines of 7 or above resulted in a 16.7% ROI profit from 84 bets. And once again, this is statistically significant and not due to random variation.

Such over-exaggeration from recent matches has forced the lines to move, and this is where we as punters can pick up on and make several value bets. In this short article, we haven’t even studied the teams and we are already picking up value bets that are statistically significant. I see no reason why this trend should not continue.

What will be interesting, is how the overs and unders work with soccer, basketball and many other sports. Will there constantly be value in all these sports? Let me know with you comments below.

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10 Comments

  1. craig eyles says:

    G’day
    I never bet o/u in any sport for the reason that there’s too many “ifs” in the totals.
    Spread betting is arguably more suitable in most cases of US sports apart from the +1.5 lines in baseball & ice hockey where the value is minimal.
    The NHL -1.5 line is currently 215-470, but at over $2 odds for every 215 wins, it’s worth it.

  2. Brett says:

    Jon, will these findings have an effect on totals handicapping for NHL?

  3. sportpunter says:

    Yes, just dont bet overs! ha, no. I dont think it will make too much difference. Because of the bias, and it is about 3%, (5% + 1%), this means that if we have a certain overlay on the overs, it may not actually exist because of the bias. Conversely, if we have a very small overlay on the unders or even not quite an overlay (not quite 5%), then the overlay might indeed be even more. Hareeba loves talking about the overs losses in the forum, I think that he especially would be interested in this article

  4. Martin says:

    Jon, what prices are used?

  5. sportpunter says:

    Official odds according to Pinnacle, usually about an hour before the start time

  6. Hareeba says:

    At what point were the prices used in these tables recorded?

  7. Hareeba says:

    sorry Jon, hadn’t seen the previous Q and A when I posted that

  8. Hareeba says:

    Quite extraordinary that it seems you can win simply backing all unders at 5.5+ lines!
    Would be interesting to know if that is replicated in say NBA and MLB.

  9. sportpunter says:

    yep a 2.25% ROI made betting under 5.5+ lines. I am checking other sports.

  10. Dwain Hespe says:

    I like how you explained this, make more sense to me now :)

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