The binomial distribution arises when n independent Bernoulli trials are conducted, each with the same probability of success p. The number of successes is a binomial(n, p) random variable.
The function ⌊x⌋ in the cdf denotes the "floor" or greatest integer function.
Parameter | Range | Description |
---|---|---|
n | n = 1, 2, ... | Number of trials |
p | 0 ≤ p ≤ 1 | Probability of success |
Probability Mass Function
Support
Mean
Variance
Example | n | p |
---|---|---|
A fair six-sided die is thrown 3 times. Let X be the number of sixes thrown. | 3 | 0.1667 |
The probability that a pregnancy in the US will result in a girl is 48.8%. A family has 7 children. Let X be the number of girls. | 7 | 0.4880 |
For a particular student, each of the 58 questions on the math SAT has an 80% chance of being answered correctly. Let X be the number of correct answers. | 58 | 0.8000 |
X ∼ Binomial(n, p)
E(X) = , Var(X) =
Note that, since a Bernoulli random variable equals 1 for a success and 0 for a failure, then a binomial random variable is the sum of n independent Bernoulli random variables. The mean and variance are therefore n times the mean and variance of a Bernoulli random variable (which are p and p(1 − p)).
Note that, since a Bernoulli random variable equals 1 for a success and 0 for a failure, then a binomial random variable is the sum of n independent Bernoulli random variables. The mean and variance are therefore n times the mean and variance of a Bernoulli random variable (which are p and p(1 − p)).
The illustration above shows a set of n independent Bernoulli(p) trials. The random variable X is the total number of successes, which has a binomial(n, p) distribution.
The simulation above shows a set of n independent Bernoulli(p) trials. Successful trials are shown in green and failures in grey. The random variable X is the number of successes, which has a binomial(n, p) distribution. The histogram accumulates the results of each simulation.