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AP Statistics
6.3A Notes
Binomial setting
A binomial setting arises when we perform several independent trials of the same chance process and record
the number of times that a particular outcome occurs. The four conditions for a binomial setting are
 Binary? The possible outcomes of each trial can be classified as “success” or “failure.”
 Independent? Trials must be independent, that is, knowing the result of one trial must not have any
effect on the result of any other trial.
 Number? The number of trials n of the chance process must be fixed in advance.
 Success? On each trial, the probability p of success must be the same.
Binomial random variable and binomial distribution
The count X of successes in a binomial setting is a binomial random variable. The probability distribution of X
is a binomial distribution with parameters n and p, where n is the number of trials of the chance process and p
is the probability of a success on any one trial. The possible values of X are the whole numbers from 0 to n.
Example
For each of the following situations, determine whether the given random variable has a binomial distribution.
Justify you answer.
1. Shuffle a deck of cards. Turn over the top card. Put the card back in the deck, and shuffle again. Repeat
this process 10 times. Let X = the number of aces you observe.
2. Choose three students at random from your class. Let Y = the number who are over 6 feet tall.
3. Flip a coin. If it’s heads, roll a 6-sided die. If it’s tails, roll an 8-sided die. Repeat this process 5 times. Let
W = the number of 5s you roll.
4. Each child of a particular pair of parents has probability 0.25 of having type O blood. Genetics says that
children receive genes from each of their parents independently. If these parents have 3 children, the
count X of children with type O blood is a binomial random variable with n = 3 trials and probability p =
0.25 of a success on each trial. In this setting, a child with type O blood is a “success” (S) and a child with
another blood type is a “failure” (F). What’s the probability distribution of X?
Binomial coefficient
The number of ways of arranging k successes among n observations is given by the binomial coefficient
n
n!
 k   k ! n  k ! for k = 0, 1, 2, …, n where n!  n  n  1 n  2  ...  3 2 1 and 0! = 1.


 
The binomial coefficient is a combination
Binomial probability
If X has the binomial distribution with n trials and probability p of success on each trial, the possible values of X
are 0, 1, 2, …, n. If k is any one of these values,
Example
To introduce her class to binomial distributions, Mrs. Desai gives a 10-item, multiple choice quiz. The catch is,
students must simply guess an answer (A through E) for each question. Mrs. Desai used her computer’s
random number generator to produce the answer key, so that each possible answer has an equal chance to be
chosen. Patti is one of the students in this class. Let X = the number of Patti’s correct guesses.
5. Show that X is a binomial random variable.
6. Find P(X = 3). Explain what this result means.
7. To get a passing score on the quiz, a student must guess correctly at least 6 times. Would you be surprised
if Patti earned a passing score? Compute an appropriate probability to support your answer.