# PDF Statistics Probability Odds Probability

It takes about 15, flips for the probability to settle at the true probability of 0.

However, note that the estimates can be very far off from the true value when the sample sizes are small. Early in the evening the vote counts were especially volatile, swinging from a large initial lead for Jones to a long period where Moore had the lead, until finally Jones took the lead to win the race.

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These two examples show that while large samples will ultimately converge on the true probability, the results with small samples can be far off. Unfortunately, many people forget this and overinterpret results from small samples. This was referred to as the law of small numbers by the psychologists Danny Kahneman and Amos Tversky, who showed that people even trained researchers often behave as if the law of large numbers applies even to small samples, giving too much credence to results from small datasets.

We will see examples throughout the course of just how unstable statistical results can be when they are generated on the basis of small samples. This reflects the use of yet another approach to computing probabilities, which we refer to as classical probability. In this approach, we compute the probability directly based on our knowledge of the situation. Classical probability arose from the study of games of chance such as dice and cards.

He expected to win money on both of these gambles, but he found that while on average he won money on the first gamble, he actually lost money on average when he played the second gamble many times. To understand this he turned to his friend, the mathematician Blaise Pascal, who is now recognized as one of the founders of probability theory. How can we understand this question using probability theory?

## Common pitfalls in statistical analysis: Odds versus risk

No loaded dice allowed! Given this, we can compute the probability of any individual outcome as:. How do we compute the probability of a complex event which is a union of single events , like rolling a one on the first or the second throw? Yet, while he consistently won money on the first bet, he lost money on the second bet. What gives? The first is the rule of subtraction , which says that:. A second rule tells us how to compute the probability of a conjoint event — that is, the probability of both of two events occurring.

This version of the rule tells us how to compute this quantity in the special case when the two events are independent from one another; we will learn later exactly what the concept of independence means, but for now we can just take it for granted that the two die throws are independent events. The addition rule tells us that:. In a sense, this prevents us from counting those instances twice.

According to our rules:. Cells shown in light blue represent the cells with a one in either the first or second throw; the rest are shown in dark blue. If you count up the cells in light blue you will see that there are 11 such cells. Thus, rather than computing the probability of at least one six in four rolls, he instead computed the probability of no sixes across all rolls:.

He then used the fact that the probability of no sixes in four rolls is the complement of at least one six in four rolls thus they must sum to one , and used the rule of subtraction to compute the probability of interest:. The probability of this outcome was slightly below 0. We often want to be able to quantify the probability of any possible value in an experiment.

For example, on Jan 20 , the basketball player Steph Curry hit only 2 out of 4 free throws in a game against the Houston Rockets. We can determine this using a theoretical probability distribution; during this course we will encounter a number of these probability distributions, each of which is appropriate to describe different types of data. This distribution is defined as:. This refers to the probability of k successes on n trials when the probability of success is p.

The binomial coefficient is computed as:. Which just goes to show that unlikely things do actually happen in the real world. Often we want to know not just how likely a specific value is, but how likely it is to find a value that is as extreme or more than a particular value.

To answer this question, we can use a cumulative probability distribution; whereas a standard probability distribution tells us the probability of some specific value, the cumulative distribution tells us the probability of a value as large or larger or as small or smaller than some specific value. In the free throw example, we might want to know: What is the probability that Steph Curry hits 2 or fewer free throws out of four, given his overall free throw probability of 0. To determine this, we could simply use the the binomial probability equation and plug in all of the possible values of k:.

In many cases the number of possible outcomes would be too large for us to compute the cumulative probability by enumerating all possible values; fortunately, it can be computed directly. For the binomial, we can do this in R using the pbinom function:.

## The Math Behind Betting Odds & Gambling

From this we can see that the probability of Curry landing 2 or fewer free throws out of 4 attempts is 0. So far we have limited ourselves to simple probabilities - that is, the probability of a single event or combination of events. However, we often wish to determine the probability of some event given that some other event has occurred, which are known as conditional probabilities. There are two simple probabilities that we could use to describe the electorate.

That is, we want to know the probability that both things are true, given that the one being conditioned upon is true. It can be useful to think of this is graphically. Every year, the survey examines a sample of about people across the US using both interviews and physical and medical tests. It also provides us with a large, realistic dataset that will serve as an example for many different statistical tools. The first Diabetes asks whether the person has ever been told that they have diabetes, and the second PhysActive records whether the person engages in sports, fitness, or recreational activities that are at least of moderate intensity.

We then use that value to compute the conditional probability, where we find that the probability of someone having diabetes given that they are physically inactive is 0.

This can be expressed as:. That is, the probability of A given some value of B is just the same as the overall probability of A. For example, there is currently a move by a small group of California citizens to declare a new independent state called Jefferson, which would comprise a number of counties in northern California and Oregon.

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That is, while independence in common language often refers to sets that are exclusive, statistical independence refers to the case where one cannot predict anything about one variable from the value of another variable. NHANES includes two relevant questions: PhysActive , which asks whether the individual is physically active, and DaysMentHlthBad , which asks how many days out of the last 30 that the individual experienced bad mental health.

The odds are usually presented as a ratio.

For example, the odds of your favorite football team losing a match may be 1 to 5. The odds of you winning a lottery might by 1 to 10, On the other hand, the odds of the horse you bet on winning the race may be equal to 4 to 3.

What do these numbers mean? There are two types of odds ratios: "odds of winning" and "odds of losing". For odds of winning, the first number are the chances for success and the second is the chances against success of losing. For "odds of losing", the order of these number is switched. Let's analyze one of these options more closely. For example, if the odds for a football team losing are 1 to 5, it means that there are 5 changes of them winning and only 1 of them losing. That means that if they played 6 times, they would win 5 times and lose once. Our betting odds calculator takes a step further and calculates the percentage probability of winning and losing.

## How to Find the Probability of an Event and Calculate Odds, Permutations and Combinations

The team would win 5 out of 6 games and lose 1 of them. Odds Calculator can be embedded on your website to enrich the content you wrote and make it easier for your visitors to understand your message. Get the HTML code. Omni Calculator logo. Embed Share via. Chances for success. Chances against success. Probability of winning. Probability of losing.