Freund, John E.; Mathematical Statistics, 5th Ed.; Prentice Hall, NJ, 1992. pp185-190. A common example of a binomial random variable is the probability of getting a certain
number on the roll of a die. It is fairly easy to derive the distribution function. The probability of getting say a 1 whenever a die is rolled is one chance in six, which is 1/6 or 0.1667. The probability of
getting five 1's in eight rolls of a die is the probability of getting five 1's and three "non-1's" in any order. The probability of getting five 1's is (0.1667)
5
. The probability of three non-1's is (1-0.1667)
3. The probabablity of getting these results in a particular order is (0.1667)
5 x (1-0.1667)
3. If all of the die were different
colors, the total number of ways you could arrange them would be 8 x 7 x .... x 2 x 1 = 8! since you can put any one of the 8 die in the first position, and for each of these any one of tne 7 remaining die in
the second position, etc. Since order does not matter here, you must divide 8! by all of the duplicate patterns. The duplicate patterns are all the ways you can arrange the 1's, which is 5!, and all the ways you
can arrange the non-1's, which is 3!. This shows that the number of ways to roll five 1's in eight rolls of a die is: 8! x 0.1667
5 x (1-0.1667)
3
/ (5! x 3!). This is the case of a binomial distribution with the probability of a success (rolling a 1) being
Q = 01667, the number of successes
X=5, and the number of trials T = 8. It is easy to
generalize this for any probability
Q, number of successes
X and number of trials
T as: T! x Q
T x (1-
Q)
1-T / [
X! x (
T-
X)!]