Calculating conditional probability. Conditional probability explained visually. Conditional probability using two-way tables.

## Pipedrive Deal Probability: Supercharging Your Sales Reporting

Conditional probability tree diagram example. Tree diagrams and conditional probability. Conditional probability and independence. Analyzing event probability for independence. Dependent and independent events. Current time: Video transcript Voiceover: Rahul's two favorite foods are bagels and pizza. Let A represent the event that he eats a bagel for breakfast and let B represent the event that he eats pizza for lunch. Fair enough. On a randomly selected day, the probability that Rahul will eat a bagel for breakfast, probability of A, is. Let me write that down.

### Notation of Probability

So the probability that he eats a bagel for breakfast is 0. The probability that he will eat a pizza for lunch, probability of event B So the probability of Let me do that in that red color. The probability of event B, that he eats a pizza for lunch, is 0. And the conditional probability, that he eats a bagel for breakfast given that he eats a pizza for lunch, so probability of event A happening, that he eats a bagel for breakfast, given that he's had a pizza for lunch is equal to 0.

So let me write this down. The probability of A given, given that B is true. Given B, is not 0. And because these two things are not the same, because the probability of A by itself is different than the probability of A given that B is true, this tells us that these two events are not independent. That we're dealing with dependent probability. This shows us. The fact that B being true has changed the probability of A being true, this tells us that A and B are dependent.

And so when we start thinking Well actually let's just, before I start going on my little soapbox about dependent events, let's just think about what they actually want us to figure out. So the probability, the probability of A given B is equal to 0. Based on this information, what is the probability of B given A?

## Joint Probability Definition

So they want us to figure out the probability of B given Probability of B given A. That's what they want us to figure out. The conditional probability that Rahul eats pizza for lunch, given that he eats a bagel for breakfast, rounded to the nearest hundredth. So how would we think about this? And I encourage you to pause this video before I work through it. So I'm assuming you've given a go at it. So the best way to tackle this is to just think about, well, what's the probability that both A and B are going to happen?

Well, the probably of A and B happening And let me do this in a new color. The probability of A and B happening. A and B. I want to stay true to the colors.

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Is equal to There's a couple of ways you could write this out. This is equivalent to, this is equivalent to the probability, probability of A given B. Given B, times the probability of B. Who likes it: Philosophers, economists, mathematicians. Which of these two ways of looking at probability is closer to statistics? The Frequency View, because it talks about collecting data.

A very important part of the Frequency View definition is that you need to repeat the same exact experiment to find the probability.

It is almost never possible where humans are concerned, for example, in sports or medicine. I would like to offer you several quotes, and you can find and correct the errors in them. Sounds like fun. When I learn to do it, I can find quotes in journals or on TV and correct them, too! I tell you, the probability of us winning the next game is 3 out of 4! Maybe the opposing team will be much stronger than usual next time. Maybe the weather will be different.

Maybe a key player will be sick. And so on. Also, the team may always win against a particular team the one that is going to play tomorrow , which will affect the chances. There is no way for a person to know her exact chances in anything that is connected with health.

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Studies show that body proportions, diet, weight, clothes preferences, number of pregnancies and breastfeeding all affect breast cancer rates in women. Even though "one out of eight" is the average for the USA, it does not tell much about each particular person. If we randomly select 1,, women and look at their medical histories, we can expect about , not exactly! Your last accident was 3 years ago, so you can expect an accident any time now. National average says close to nothing about your chances of having an accident.

If you randomly choose drivers, you can expect them all together to have had about accidents over the previous 10 years. All these errors are of the same type. They take data about large numbers of people, and try to use it in personal cases. Collecting data about large numbers of people or other objects , and using this data for studying other large groups of people as you did in the "Conclusion that may be true" column, belongs to statistics. The only time it can be used for probability, that is, for studying chances in individual cases, is when all the experiments are the same or almost the same.

You can use data statistics from rolling a six-sided die one million times in exactly the same manner! You can not use data statistics from studying driving records of a million people to find the chances probability of yourself having an accident today. So statistics deals with data that may or may not be useful for finding probability. Data can also be useful by itself, without any connection to probability. For example, you need to know, at least approximately, how many voters live in a particular city in order to prepare for elections.