# Conditional Probabilities

## Understanding Conditional Probability and Its Applications

On a cloudy day with a chance of rain, there may be a need to run an errand outside. This situation highlights the concept of conditional probability, where the likelihood of both going outside and experiencing rain must be taken into consideration. In this article, we will explore the concept of conditional probability and its practical applications.

## What is Conditional Probability?

Conditional probability refers to the probability of an event occurring based on certain conditions. One event is typically dependent on another event, meaning the latter's occurrence relies on the former. To determine the probability of the dependent event, we first need to consider the probability of the initial event. While the first event follows regular probability rules, the second event is referred to as the conditional probability.

The probability of event B happening, given that event A has already occurred, is known as conditional probability. In other words, event A acts as a condition for event B to happen. It is important to note that this does not necessarily mean that both events happen simultaneously, nor do they always have a causal relationship.

- For instance, imagine a box with red, white, and black balls. If we are asked to find the probability of getting a black ball after getting a red ball, event A would be getting the red ball, and event B would be getting the black ball, which is dependent on event A.
- On the other hand, independent events are those where the likelihood of one event does not affect the other. The result of these events has no impact on each other. Therefore, the probability of independent events is merely the product of the probabilities of each individual event. In cases where event B is dependent on event A, these events are known as dependent events. As there is a condition applied to finding the probability, the formula for dependent events includes a conditional probability term.

## The Conditional Probability Formula

The conditional probability formula calculates the probability of event B based on the occurrence of event A. It is represented as:

**P(B|A) = P(A and B) / P(A)**

The formula is derived from the fact that the probability of an event is the number of favorable outcomes divided by the total number of outcomes. P(S) represents the probability of the sample space. In cases where both events A and B are independent, the conditional probability simplifies to the probability of event B.

Therefore, we can state that: **P(B|A) = P(B)**, as independent events have no effect on each other.

## Properties of Conditional Probability

The properties of conditional probability are all based on the formula mentioned above, where P(S) is the probability of the sample space and P(A) and P(B) are the probabilities of events A and B respectively. Some key properties include:

- If P(X) is any event where P(B) > 0, then P(X|B) = P(X and B) / P(B)
- If P(B') = 1, then P(B'|A) = 1 - P(B|A)
- If P(A and B) = 0, then P(A|B) = 0

## Other Methods of Calculating Conditional Probability

Aside from the formula, there are two other methods that can be used to calculate conditional probability:

- Tree Diagram: This method is helpful in visualizing and solving conditional probability problems. A tree-like structure is drawn to represent all events. Typically, for the first event (event A), two branches are drawn, and for the second event (event B), four branches are drawn.
- Venn Diagram: Another method used to solve conditional probability problems is the Venn diagram, which requires the probabilities of event A, event B, and their intersection. In the diagram, event A and event B are represented as circles within the sample space, with their intersecting part representing the event A and B. It is important to note that the sum of all probabilities in a Venn diagram should always equal 1.

## Understanding Conditional Probabilities

Conditional probability is a concept used in mathematics to determine the likelihood of an event B happening, given that an event A has already occurred. This concept has significant applications in probability and statistics.

## Examples of Conditional Probability

To better grasp conditional probability, let's consider a few examples. Imagine Leah, a student who excels in mathematics. The probability of her studying for a math exam and passing it is , while the probability of her studying for the exam is . Using this information, we can calculate the probability of Leah passing the exam given that she studied for it.

**Solution:** Let event S represent Leah studying for the math exam and event P represent her passing the exam. With the probability of her studying and passing the exam being , and the probability of her studying being , we can use the conditional probability formula to determine the desired probability.

**Therefore, the probability of Leah passing the math exam given that she studied for it is 0.91.**

## Conditional Probability Illustrated with Tree Diagrams

A tree diagram is another effective way to demonstrate conditional probability. For example, let's say there is a box containing 10 strawberry candies (S) and 10 chocolate candies (C). If Nova randomly picks a candy after Ava has already selected a strawberry candy, what is the probability of Nova also picking a strawberry candy?

**Solution:** To solve this, we first note that there are 20 candies in the box, half being strawberry and the other half being chocolate. We can use a tree diagram to visualize the two random selections made by Ava and Nova.

- Probability of Ava getting S:
- Probability of Ava getting C:

Now, for Nova's selection, we consider the possible outcomes based on Ava's selection. If Ava chose S, the probability of Nova getting S is and the probability of C is . If Ava chose C, the probability of Nova getting S is and the probability of C is . This can be represented in a tree diagram as follows:

**Probability of Nova getting S after Ava got S:**

**Probability of Nova getting S after Ava got C:**

**Therefore, the probability of Nova picking a strawberry candy after Ava has already selected one is: **

## Conditional Probability Demonstrated with Venn Diagrams

Venn diagrams can also be utilized to solve conditional probability problems. For example, in a class, some students study Spanish and others study French.

**Solution:** If a French student is randomly chosen, what is the probability that they also study Spanish? We can use a Venn diagram to visualize the likelihood of this event.

- French students probability:

- Spanish and French students probability:

**Therefore, the probability of a French student also studying Spanish is: **

## Conditional Probability of Two Children

Consider a family with 2 children, one of whom is a boy. What is the probability of the other child also being a boy?

**Solution:** To solve this, we can use the events b for boy and g for girl. We define event A as having one child as a boy (b) and event B as having both children as boys (bb). The sample space for these events is: {gg, gb, bg, bb} with probabilities of , , , and , respectively. To determine the desired probability, we use the conditional probability formula.

**Therefore, the probability of the other child being a boy is: **

## Essential Points to Remember

- Conditional probability is the likelihood of an event B happening, given that an event A has already occurred.
- The conditional probability formula is:
**P(B|A) = P(A and B)/P(A)** - Tree diagrams and Venn diagrams are useful tools for solving conditional probability problems.

## How to Calculate Conditional Probability?

The following steps are involved in calculating conditional probability:

- Determine the events A and B.
- Find the probability of event A,
**P(A)**. - Find the probability of both events A and B,
**P(A and B)**. - Apply the conditional probability formula to solve for P(B|A).

## The Important Concept of Conditional Probability

Probability has various types that can be calculated depending on the situation. One of these is conditional probability, which involves determining the probability of an event occurring given that another event has already happened. Understanding the average rule of conditional probability is essential in this field, as it helps us understand the relationship between different probabilities.

The average rule states that the overall probability of an event, designated event A, is equal to the weighted average of all conditional probabilities. In other words, we can calculate the probability of event A by using a combination of individual conditional probabilities.

For instance, if we want to find the probability of getting a heads on a coin flip twice in a row, we know that the probability of getting a heads on one flip is 0.5 or 50%. By applying the average rule, we can calculate the overall probability by multiplying the probability of getting a heads on the first flip (0.5) with the probability of getting a heads on the second flip, given that the first flip was a heads as well (0.5). This gives us a final probability of 0.25 or 25%. This is because the likelihood of getting both heads in two flips is lower than the likelihood of getting just one heads.

The average rule of conditional probability can also be used in more complex scenarios involving multiple events. In this case, we need to consider all the individual probabilities and their corresponding conditional probabilities. Then, we calculate the weighted average of these conditional probabilities to determine the overall probability of the event in question.

Having a solid understanding of the average rule of conditional probability is crucial in various fields, including statistics, economics, and finance. It enables us to make more accurate predictions and decisions by considering the likelihood of certain events happening in relation to one another.

So, the next time you come across a probability problem that involves multiple events, keep the average rule in mind and how it can aid you in solving it. By being familiar with the fundamental concept of conditional probability, you can elevate your understanding of probability and its applications in different areas.