ELI5: Explain Like I'm 5

Binomial approximation

Imagine you have a bag of red and green marbles. You know how many of each color there are, but you want to predict the likelihood of getting a certain number of red marbles if you were to blindly pick out a certain number of marbles from the bag.

To do this, you can use the binomial theorem to approximate the probability. The binomial theorem helps you calculate the probability of getting a certain number of "successes" (in this case, picking a red marble is considered a "success") out of a certain number of trials.

The binomial theorem formula looks like this:

P(x) = (n choose x) * p^x * q^(n-x)

Where:
- P(x) = probability of getting x "successes"
- n = total number of trials (in this case, the number of marbles you will pick from the bag)
- x = the number of "successes" you want to calculate the probability for (in this case, the number of red marbles you want to predict)
- p = probability of a "success" (in this case, the probability of picking a red marble from the bag)
- q = probability of a "failure" (in this case, the probability of picking a green marble from the bag)

So, let's say you have a bag with 20 marbles and 10 of them are red. You want to predict the likelihood of picking 3 red marbles if you blindly pick out 6 marbles.

Using the binomial theorem, you can calculate:

P(3) = (6 choose 3) * (0.5)^3 * (0.5)^3

= 20 * 0.125 * 0.125

= 0.3125

This means there is a 31.25% probability of picking exactly 3 red marbles if you blindly pick out 6 marbles from the bag.

The binomial approximation allows you to simplify this calculation if n (the number of trials) is very large or if p (the probability of success) is very small or very large. In these cases, you can use other statistical methods to approximate the probability without having to do all the complicated math.

But basically, the binomial theorem is a way to predict the probability of getting a certain number of successes in a certain number of trials, and this can be used to help you make predictions or decisions based on data.