BinomialDistribution/Revisited

The binomial distribution density function provides the probability that X success will occur in N trials of a binomial experiment.

A binomial experiment has the following properties.

• 1). It consists of a sequence of N individual trials.
• 2). Two outcomes are possible for each trail, success or failure.
• 3). The probability of success on any trial, denoted p does not vary from trail to trial.
• 4). The trials are independent.

One common example of a binomial experiment is a sequence of N tosses of a coin.

 Here:

• 1). This is clearly an experiment that consists of a sequence of N trials.
• 2). There are only two outcomes, Heads and Tails, Let call Heads a success and Tails a failure.
• 3). The probability of success on any one trail, denoted p, here ½, does not vary from trial to trial.
• 4). The trails are independent – The success of the third trial, say, does not depend on the success of the 2nd trial, etc.

To find the binomial distribution function of X successes in N trials, denoted f(X), we let p denote the probability of success on any one trial. Then the probability of failure on any one trial is q=1-p. Then the probability of any particular sequence of X success (heads) out of N trials (tosses) is p^X*q^(N-X). But, there are a variety of sequences of tosses that will result in X successes out of N trials. Then, using the formula for the number of ways of obtaining X successes out of N trials, we find that the number of ways of picking X objects out of N objects that there are

• (N!/(X!*(N-X)!) ways, where N!=N factorial.

• We will denote (N!/(X!*(X-N)!) by:

• (N)
• (X). (apologies…for symbolism)

Then the probability of N successes out of N trials is given by the FunctioN?:

       F(X)=  (N) *  p^X*q^(N-X) = (N) * p^X*(1-p)^(N-X)                                    
              (X)                  (X)

For example, consider the experiment of tossing a die with the usual 6 sides. Suppose we want to know the probability of getting three "1"s in 5 tosses. Then p=1/5. So q=4/5.

Then, using the function of the binomial distribution function for 3 successes out of 5 trials we get the probability of this occurring as:

• F(2)= (5!/(3!*2!)) * (1/5)^3 * (4/5)^2=(5*2)* (1/125)*(16/25)=(10)* (16/3023) * =160/3125=.0512