Conditional Probability Explained

When we think about probability, we often calculate the chance of an event happening out of all possibilities. But what happens if we already have some information? What if we know something else has occurred? These are the questions addressed by conditional probability.

Conditional probability measures the likelihood of an event A occurring, given that another event B has already happened. Think of it as updating our probability estimate based on new information. We write this as $P(A|B)$, which reads "the probability of A given B".

The Intuition: Reducing the Sample Space

The "given B" part is important. When we know that event B has occurred, the original sample space (the set of all possible outcomes) is no longer entirely relevant. We can effectively ignore any outcomes where B did not happen. Our focus shifts to a new, reduced sample space consisting only of the outcomes where B occurred. Conditional probability then asks: within this new, smaller context where B is true, what is the chance that A also happens?

Calculating Conditional Probability

The formal definition and formula for conditional probability is:

$$P(A|B) = \frac{P(A \cap B)}{P(B)}$$

Let's break this down:

1. $P(A \cap B)$ (The Numerator): This is the probability that both event A and event B occur. Remember from set theory, the intersection symbol ($\cap$) means "and". We need the outcomes that are common to both A and B.
2. $P(B)$ (The Denominator): This is the probability of event B occurring (our given information). This acts as the normalization factor, representing the size of our reduced sample space.
3. The Condition: This formula only makes sense if $P(B) > 0$. We cannot condition on an event that has zero probability of happening.

Essentially, the formula calculates the proportion of outcomes where A occurs out of the outcomes where B occurs.

Example 1: Rolling a Die

Let's use a standard six-sided die. The sample space $S = \{1, 2, 3, 4, 5, 6\}$.

Define two events:

• Event A: Rolling a 4. $A = \{4\}$.
• Event B: Rolling an even number. $B = \{2, 4, 6\}$.

First, let's find the probabilities we need:

• $P(B)$: There are 3 even numbers out of 6 possible outcomes, so $P(B) = 3/6 = 1/2$.
• $P(A \cap B)$: This is the probability of rolling a 4 and rolling an even number. The only outcome that satisfies both is rolling a 4. So, $A \cap B = \{4\}$, and $P(A \cap B) = 1/6$.

Now, let's find the conditional probability $P(A|B)$: "What is the probability of rolling a 4, given that we know the roll was an even number?"

Using the formula:

$$P(A|B) = \frac{P(A \cap B)}{P(B)} = \frac{1/6}{1/2} = \frac{1}{6} \times \frac{2}{1} = \frac{2}{6} = \frac{1}{3}$$

Does this match our intuition? If we know the roll was even, our reduced sample space is just $\{2, 4, 6\}$. Within this set of 3 equally likely outcomes, only one outcome is a 4. So, the probability is indeed 1/3. The conditional probability $P(A|B) = 1/3$ is higher than the original probability $P(A) = 1/6$, because knowing the number was even increased the chances of it being a 4.

Example 2: Using Survey Data

Imagine a small survey of 100 people about pet ownership and housing type. The results are summarized in the table below:

	Has Cat	No Cat	Total
Apartment	15	35	50
House	25	25	50
Total	40	60	100

Let's define events:

• Event A: A randomly selected person has a cat.
• Event B: A randomly selected person lives in an apartment.

We want to calculate $P(A|B)$: "What is the probability that a person has a cat, given that they live in an apartment?"

First, find the necessary probabilities from the table (assuming we select one person uniformly at random from the 100):

• $P(B) = P(\text{Apartment})$: There are 50 apartment dwellers out of 100 people. $P(B) = 50/100 = 0.5$.
• $P(A \cap B) = P(\text{Has Cat AND Apartment})$: There are 15 people who both have a cat and live in an apartment. $P(A \cap B) = 15/100 = 0.15$.

Now, apply the conditional probability formula:

$$P(A|B) = \frac{P(A \cap B)}{P(B)} = \frac{15/100}{50/100} = \frac{0.15}{0.50} = \frac{15}{50} = 0.3$$

So, the probability that someone has a cat, given they live in an apartment, is 0.3 or 30%.

Again, let's check the intuition. If we know the person lives in an apartment, we only care about the first row of the table. In that row, there are 50 people total, and 15 of them have cats. The proportion is $15/50 = 0.3$. The formula gives us the same result.

Relevance to Machine Learning

Conditional probability is a foundational concept in many areas of machine learning. For instance:

• Classification: Models often estimate the probability that an input belongs to a certain class given its features. $P(\text{Class} | \text{Features})$. For example, $P(\text{Spam} | \text{Email contains 'free money'})$.
• Bayesian Methods: Bayes' Theorem, which we'll introduce next, is built directly upon conditional probability and is used extensively in modeling uncertainty and updating beliefs.

Understanding how the probability of one event changes based on the occurrence of another is essential for interpreting data and building models that learn from it. It allows us to quantify how different pieces of information relate to each other.