In this explainer, we will learn how to calculate conditional probability using formulas and Venn diagrams.

Conditional probability is the probability of an event occurring **given** some knowledge about the outcome
of some other event. Depending on the form of the problem, there are a few different methods
we can use to help us calculate conditional probabilities. We are going to use Venn diagrams
and the conditional probability formula.

Before we go any further, we will need to use some of the rules of probability that you already know, so let us first remind ourselves of these.

### Some Probability Rules

For any event , if is the probability of event occurring, then we have the following:

**Rule 1**

**Rule 2**

The sum of the probabilities of all possible outcomes is equal to 1.

**Rule 3**

The probability of event NOT occurring is known as the complement, , of event , or βnot ,β is given by

This can be represented on a Venn diagram ( is the orange region):

### Note

The complement of event is sometimes also written as .

**Rule 4**

The probability of event or event occurring is known as the βunionβ of and and is given by .

This can be represented on a Venn diagram ( is the orange region):

**Rule 5**

The probability of event and event occurring is known as the βintersectionβ of and , and is given by .

This can be represented on a Venn diagram ( is the orange region):

**Rule 6**

The probability that event occurs, but event does not can be thought of as , or alternatively the intersection between and , written and represented on a Venn diagram as follows by the orange region.

**Mutually Exclusive, or Disjoint Events**

Next, let us recall the idea that events may be mutually exclusive (or disjoint). That is, the events cannot occur at the same time.

For example, an animal may either be a cat or a dog, but it cannot be both a cat and a dog. βBeing a catβ and βbeing a dogβ are mutually exclusive events.

For mutually exclusive events, the probability of the intersection of those events is zero. In our example, .

For mutually exclusive events, the probability that either of those events occur is simply the sum of their individual probabilities of occurring.

In our example, .

**Rule 7**

In general, for two mutually exclusive events and ,

- ,
- .

**Non-Mutually Exclusive Events**

For example, a person may like cats, or they may like dogs. They may even like cats and dogs. βLikes catsβ and βlikes dogsβ are not mutually exclusive events.

We need to be careful how we calculate the probability that a person likes cats OR dogs, .

If we simply added together the probability that a person likes cats and the probability that the person likes dogs, we would count the area of intersection (the probability they like both) twice. We need to make an adjustment for this in our calculation.

**Rule 8**

In general, for non-mutually exclusive events and ,

As mentioned, conditional probability is the probability of an event occurring **given**
some knowledge about the outcome of some other event.

We can mathematically define conditional probability as follows.

### Definition: Conditional Probability

If the probability of an event is affected by the occurrence of an event , then we say that the probability of is conditional on the occurrence of . The notation for conditional probability is , read as βthe probability of given .β

There are many situations where we might apply the rules of conditional probability. For example, the probability that a personβs eyes are a particular color is affected by, or conditional on, the color of their parentsβ eyes. Similarly, the time a journey takes is likely to be conditional on the mode of transport used. The value of your second card from a pack, if you do not replace the first, is conditional on the value of your first. These are all situations where conditional probability can be applied. We can define conditional probability mathematically as follows.

Let us look at an example involving two non-mutually exclusive events. This leads us to a situation where the probability of one event is conditional on the probability of the other.

### Example 1: Conditional Probability with Venn Diagrams

On the street, 10 houses have a cat (C), 8 houses have a dog (D), 3 houses have both, and 7 houses have neither.

- Find the total number of houses on the street. Hence, find the probability that a house chosen at random has both a cat and a dog. Give your answer to three decimal places.
- Find the probability that a house on the street has either a cat or a dog or both. Give your answer to three decimal places.
- If a house on the street has a cat, find the probability that there is also a dog living there.

### Answer

**Part 1**

Let us begin by putting the information we have into a Venn diagram. The left oval will contain houses with cats, the right oval will contain houses with dogs. The intersection of the ovals contains houses with both cats and dogs.

In examples like these, it is helpful to consider the intersection first. Although 10 houses have cats, some of them may also have dogs. Although 8 houses have dogs, some of them may also have cats. Since we are told that 3 houses have both cats and dogs, we can fill in this area of our Venn diagram first (the intersection of houses with cats and houses with dogs).

Now, if 10 houses have cats, and we know that 3 of them also have dogs, this leaves houses that only have cats.

Likewise, if 8 houses have dogs, but 3 of those houses also have cats, then houses just have dogs.

Finally, we are told that 7 houses have neither cats nor dogs, so we can fill in the area outside the ovals.

We can find the total number of houses by adding the numbers from each section of the diagram. So the total is . We could also calculate this without using the Venn diagram of course, using the numbers we are given in the question: . For this last calculation, notice the 3 houses with both a cat and a dog have been subtracted, because these have been counted twice: once within the 10 cat houses and again within the 8 dog houses.

Let us define C as being the event that a house has a cat and D as the event that a house has a dog.

As there are 3 houses with both a cat and a dog and 22 houses in total, the probability that a house has both a cat and a dog is then .

Hence the probability is 0.136 to three decimal places.

**Part 2**

To find the probability that a house has either a cat or a dog or both, we can use the rule of probability that . Hence,

Hence, the probability that a house has either a cat or a dog or both is 0.682 to three decimal places.

**Part 3**

When answering the final part of this question, we must recognize that the phrase βif a house on the street has a catβ is setting a condition. By doing this, we are restricting the cases we are looking at to include only the 10 houses that own a cat.

Out of the 10 houses we are now considering, only 3 of them have dogs, so if a house on the street has a cat, then the probability it also has a dog is .

In part (3), we have actually calculated the conditional probability, : the probability that a house has a dog (D), given that it has a cat (C).

By using a Venn diagram in our previous conditional probability example, we could see that when we are βgiven a conditionβ, it determines a subset of cases for us to consider. Being told that the house on the street had a cat meant that we were only looking at a subset of 10 houses. Further, when looking for houses in our subset that had a dog, we could see that these houses must have both a cat and a dog.

Thinking of probability as a proportion of cases that match a given condition, we can see
that the probability that a house has a dog **given** that it has a cat can be expressed as

This might be thought of as βWhat proportion of houses with cats actually had cats and dogs?β

This gives us a general conditional probability formula for events and

Let us now try an example using the conditional probability formula.

### Example 2: Using the Conditional Probability Formula

Suppose and are two events. Given that and , find .

### Answer

Recall the conditional probability formula

However, in our question, we have been asked to find , so letβs reverse and in the general formula:

Further, by visualising the Venn diagram, it is clear that .

The orange region of intersection can be described as either , or .

So, to find the probability of given , we can use the formula

Substituting in the values we have been given,

The probability is therefore .

An important idea in probability theory is the independence of events.

If we repeatedly flip a coin to see if it lands head side up, or tail side up, each flip is an independent event. What happens in one flip does not affect the probabilities of heads or tails in any other flip.

Now consider the situation where we have a bag containing 5 lemon sweets and 5 chocolate
sweets, and I take out one sweet at random and eat it, then the probability of getting a lemon
sweet is , and the probability of getting a chocolate
sweet is also . However, the probability of getting
each flavour will be different for the next person to take one at random. If I ate a lemon
sweet, then the next person has a probability of of selecting a
lemon sweet, and a probability of selecting a chocolate sweet.
However, if I ate a chocolate sweet, then the next person has a probability of
of selecting a lemon sweet, and only
probability of selecting a chocolate sweet. The probabilities
of the next person picking each flavour of sweet are **dependent** upon what flavor I
picked.

### Definition: Independent and Dependent Events

Two events and are

**independent**if the fact that occurs does not affect the probability of occurring,**dependent**if the fact that occurs does affect the probability of occurring.

And we have the following.

**Rule 9**

For **independent** events, .

But if we rearrange the formula for conditional probability, we find that

Comparing with rule 9, this must mean that, for independent events, and similarly, for , that

This seems logical, as it is saying that for independent events, the probability that occurs given that occurs is the same as the probability that simply occurs (whether, or not, occurs). In other words, the probability that occurs is the same, regardless of the outcome of event .

Usefully, this gives us a way of checking whether two events are independent.

### Test for Independence of Events π΄ and π΅

If and , then events and are independent.

It is important to note that **both** of these equations must be true for
independence.

Let us look at an example.

### Example 3: Determining Whether Two Events Are Independent Using Conditional Probability

Suppose and . The probability that event occurs and event also occurs is . Calculate , and then evaluate whether events and are independent.

### Answer

To find the probability of given , we can use the formula

Substituting in the values we have been given, we have

If events and are independent, then one of the equations that must be true is .

This is not the case since and Since , events and are not independent.

### Example 4: Using Venn Diagrams to Calculate Dependent Probabilities

The figure shows a Venn diagram with some of the Probabilities given for two events and .

- Work out .
- Work out .
- Work out .

### Answer

Let us first define variables to represent any missing probabilities in our Venn diagram. In this case there is just one missing probability, so let us use to represent it.

We know that the probabilities on our Venn diagram must sum to one, as they represent all possible outcomes (rule 2). We can therefore calculate the value of by subtracting the sum of the other probabilities from one:

Now we have a complete Venn diagram, which will help us to answer the questions.

**Part 1**

means the probability of a result in the region defined by the intersection of region and the region βnot β. First, let us shade region :

Then let us shade region βnot β:

So the region consists of areas shaded twice:

This is the region we defined with the probability , so .

**Part 2**

To find , we simply need to sum the probabilities of the regions making up region .

**Part 3**

is the probability that event occurs given that event occurs. To find using our Venn diagram, we could ask ourselves βwhat proportion of the region representing event contains cases where event also occurs?β

Let us first restrict the cases to the region where event has occurred. In fact, this can be illustrated by the same diagram we drew in part 2. Now, given that we are only considering events in this shaded region, letβs further shade the subset of this region in which event occurs.

So,

We could check our answer using the conditional probability formula .

From the Venn diagram, we can see that , and . Substituting these values into the conditional probability formula gives us , which again simplifies to or 0.667 to three decimal places.

Let us finish by recapping on some key points.

### Key Points

- If the probability of an event is affected by the occurrence of an event , then we say that the probability of is conditional on the occurrence of . The notation for conditional probability is , read as the probability of given .
- For two events, and , we can use Venn diagrams, and the conditional probability formula to help work out the conditional probability of given . The formula is given by
- We can use conditional probabilities to help us determine whether two events are dependent or independent.
- Events and are independent if and