Written by Clay Smith
The odds for favorites are accompanied by a minus (-) sign, indicating the amount you need to stake to win $100. Meanwhile, the odds for underdogs are accompanied by a positive (+) sign, indicating. Betfair odds include the stake in their odds, so you have to subtract 1 to get to normal bookie odds. By this I mean, when a bookie says 'evens', he would write that either as 'eve, or 1/1'. In Betfair parlance, you would say, 'ah, well, 1/1 = 1, so Betfair would show 1.00'.
Idiot’s Guide
That’s right - I will be your guide. The good thing about having an idiot for a guide is that I have to make it simple to understand it myself, which means, hopefully, you will understand it as well.
Probability or Odds
Probability
Probability means the risk of an event happening divided by the total number of people at risk of having that event. I will use the example in a recent JAMA article. In a deck of 52 cards, there are 13 spades. So, the risk (or probability) of drawing a card randomly from the deck and getting spades is 13/52 = 0.25 = 25%. The numerator is the number of spades, and the denominator is the total number of cards.
Odds
Odds seems less intuitive. It is the ratio of the probability a thing will happen over the probability it won’t. In the spades example, the probability of drawing a spade is 0.25. The probability of not drawing a spade is 1 - 0.25. So the odds is 0.25/0.75 or 1:3 (or 0.33 or 1/3 pronounced 1 to 3 odds).
Moving back and forth
To go from odds to probability, simply take the numerator/(denominator + numerator). In the spades example, the odds of 1/3 is converted by taking 1/1+3 = 0.25 - and now we are back to probability. To go from probability to odds, simply take the numerator/(denominator-numerator). In the spades example, given that the probability of drawing a spade is 1/4, take 1/(4-1) = 1:3 odds or odds = 0.33.
Statistical Significance
If an odds ratio (OR) is 1, it means there is no association between the exposure and outcome. So, if the 95% confidence interval for an OR includes 1, it means the results are not statistically significant. Example, exposure to colored vs white Christmas lights was associated with an increase in jocularity score, OR = 1.2 (95%CI 0.98-1.45). Sorry, this is not statistically significant. Let’s just go with white lights…
Use
Either the OR or risk ratio (RR) could be used in many study types. However, only the OR can be used in case-control studies. Because in order to calculate the RR, one must know the risk. Risk is a probability, a proportion of those exposed with an outcome compared to the total population exposed. This is impossible in a case-control study, in which those who already have the outcome are included without knowing the total population exposed.
Risk Ratio
RR is a very intuitive concept. It is the probability (or risk) of one outcome over the probability (risk) of another. Let’s use a study we covered on JF to discuss this concept. Survival was lower in pediatric patients intubated during arrest compared with those not intubated: 411/1135 (36%) vs 460/1135 (41%). So, the RR is 36.2%/40.5% = 0.89. This means survival was reduced by a factor of 0.89 for pediatric arrest patients who were intubated during arrest vs. those who were not. As an example, if survival was expected to be 40%, then intubating during arrest would reduce it to: 40% x 0.89 = 35.6%.
Let’s do one more example. Supination-flexion (SF) vs hyperpronation (HP) to reduce nursemaid’s elbow was more likely to fail. The risk of failure with SF was 96/351 (27%) vs. 32/350 (9%) with HP. The RR was 3. This has a very intuitive meaning: risk of failure with SF was three times more likely than HP.
Odds Ratio
The OR is a way to present the strength of association between risk factors/exposures and outcomes. If the OR is <1, odds are decreased for an outcome; OR >1 means the odds are increased for a given outcome. Let’s look at the examples again and consider odds.
For pediatric arrest, the risk of survival if intubated during arrest was 411/1135 (36%) vs 460/1135 (41%) if not intubated. Let’s convert to odds and calculate an OR.
Intubated: 411/1135-411 = 411/724 = 0.57 odds.
Non-intubated: 460/1135-460 = 460/675 = 0.68 odds.
So, the OR is 0.57/0.68 = 0.83.
Note, this is very close to the RR (0.89) but is a slight overestimate of the effect on the outcome. This is always the case with the OR compared to the RR - it overestimates the effect.
Take the example of supination-flexion vs hyperpronation for nursemaid’s. The risk of failure for SF was 96/351 vs. 32/350 with HP. Let’s convert this to odds.
SF: 96/351-96 = 0.376 odds
HP: 32/350-32 = 0.10 odds
The OR is 0.376/0.10 = 3.7
Note, the OR overestimates the RR, which was 3. Although one could say the risk of failure using SF is 3 times greater than HP, one could not say, based on the OR, the risk was 3.74 times greater. The OR and RR are not the same. What could be said is that the odds of failure is 3.74 times greater.
Risk Ratio vs Odds Ratio
Whereas RR can be interpreted in a straightforward way, OR can not. A RR of 3 means the risk of an outcome is increased threefold. A RR of 0.5 means the risk is cut in half. But an OR of 3 doesn’t mean the risk is threefold; rather the odds is threefold greater. Interpretation of an OR must be in terms of odds, not probability. Again, the OR will always be an overestimate compared to the RR. However, the RR and OR will be similar for rare outcomes, <10%. But the OR increasingly overestimates RR as outcomes exceed 10%. This is easier to understand with an example.
Pretend a new vape, Vapalicious, is associated with cancer.
80/100 people who use it get cancer.
20/100 who don’t use it get cancer.
The risk of getting cancer is 4 times greater in Vapalicious users. RR = 0.8/0.2 = 4
Note how distorted the OR becomes in this example. OR = (80/20)/(20/80) = 16
What if Vapalicious rarely caused cancer?
5/1000 get cancer with Vapalicious vs 2.5/1000 for non-users.
RR = 2.
OR = 2 as well (actually 2.005)
With rare outcomes, the RR and OR are very similar.
Why Does This Matter?
This matters because we often equate the OR and RR. Unwary researchers, reviewers, or news media might report a 16-fold increased risk of cancer from Vapalicious. In fact, there was a 4-fold increased risk of cancer from Vapalicous. Not that I plan to use Vapalicious (or any other vape), but a 16-fold vs 4-fold increase is a gross overestimation of the effect.
What Does the OR Mean?
So, what does an OR mean? Here it is in plain language.
An OR of 1.2 means there is a 20% increase in the odds of an outcome with a given exposure.
An OR of 2 means there is a 100% increase in the odds of an outcome with a given exposure. Or this could be stated that there is a doubling of the odds of the outcome. Note, this is not the same as saying a doubling of the risk.
An OR of 0.2 means there is an 80% decrease in the odds of an outcome with a given exposure.
Summary
Odds Ratio is a measure of the strength of association with an exposure and an outcome.
OR > 1 means greater odds of association with the exposure and outcome.
OR = 1 means there is no association between exposure and outcome.
OR < 1 means there is a lower odds of association between the exposure and outcome.
If the 95% confidence interval for the OR includes 1, the results are not statistically significant.
OR and RR are not the same.
OR always overestimate RR, but…
OR approximates RR when the outcome is rare but markedly overestimates it as outcome exceeds 10%.
References
The odds ratio by Bland and Altman, of Bland-Altman plot fame
Wikipedia aka source of all statistical knowledge
Welcome to the Sports Betting Odds section of The Sports Geek. If you are new to sports betting and don’t understand how to read betting odds (+150, -110, +2200, etc) we will lay it all out for you and help you learn how the betting odds work.
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Sports Betting Odds Explained
Many people don’t know how to read or calculate sports betting odds, so below we have done our best on explaining how betting odds work. The most common type of sports betting odds used in North America are the American style odds which we explain below.
American Style Sports Betting Odds
Most online sportsbooks will list their odds in what is called “American Odds”. There are a couple different versions of sports betting odds, but these American Odds are the most common odds used. Reading and understanding sports betting odds can bet a little confusing to beginners, so we have provided an example below using two NFL football teams:
American Odds
- Matchup Odds
- New England Patriots +120
- Pittsburgh Steelers -140
The number shown in the bracket represents the odds. The American Odds have two components to them, the first being the positive or negative sign, and the second being the number that follows the sign.
The sign in front of the number indicates whether placing a wager on that outcome will pay out more money then you have wagered or less money then you have wagered. If the odd is negative (-) it means that outcome is more likely to happen and placing a bet on that outcome would payout less than the amount you wagered, while a positive (+) odd shows that the outcome is less likely to happen and it would pay out more than the amount you wagered.
100 1 Odds Meaning Math
The next step is figuring out exactly how much the bet pays out, which is where the numbers in the odds come into play.
A listed odd with a – sign in front of it, such as the -140 in our example above, shows us how much money you would need to wager in order to win $100. So using the -140, this would show us that you would need to bet $140 in order to win $100 in profits. You can easily substitute the $100 bet for a $10 bet by moving the decimal place over one spot, showing us that you would need to wager $14 in order to win $10 in profits.
Examples:
A listed odd with a + sign in front of it, such as the +120 in our example above, shows us how much money you would win on a $100 bet. Using the +120 odds, it shows us that a $100 bet on that outcome would pay out $120 in profits. Again this can easily be converted into smaller or larger size bets. A $10 bet on +120 odds would pay out $12 in profits.
Examples:
Below is an example of NFL betting odds taken from an online betting site.
In this example you can see Los Angeles is listed at +130 ($100 bet pays $130 plus of course your original wager back) and New England is listed at -150 ($150 bet pays $100).
The great thing about betting online is that the online sportsbooks will do the calculations for you before you place your bet. You can click on the outcome or team you would like to bet on, and then input the amount you wish to wager and it will show you your potential pay out before you confirm your bet.
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Decimal Style Sports Betting Odds
Decimal style odds are used mostly in Europe, and are pretty easy to understand. To calculate the decimal style odds all you will need to do is simply multiply the amount you wish to wager by the decimal odds shown and you will get your payout. For example it may look something like this:
100/1 Odds Meaning
If you wanted to place a $10 wager on the USA at 2.40 you would simply need to multiply your $10 wager by the 2.40 odds (10 x 2.40) to find out that the payout is $24. It is important to realize that with decimal style odds it includes the amount you wagered, so to find out profits you would need to subtract your wager ($24 – $10) to find out your potential payout is $14 in profits.
If you wanted to place a $10 wager on Brazil you would again just multiply $10 x 1.55 to find out that you would win $15.50 total or $5.50 in profits.
100 1 Odds Meaning In Golf
Decimal style betting odds are very simple to understand, but you won’t see them displayed in many North American sportsbooks. With that said, most online betting sites will allow you to chose the style of betting odds you want displayed, with American odds set as the default.