Poisson Approximation of Binomial Probabilities
|
TP(k out of n) = | (enp)(npk) k! |
e = | the base of the natural logarithms;
n =
| the number of opportunities for event x to occur;
| k =
| the number of times that event x occurs or is stipulated to occur; and
| p =
| the probability that event x will occur on any particular occasion;
| |
n | k | p | q
|
binomial mean | binomial SD | discrepancy | |
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The defining characteristic of a Poisson distribution is that its mean and variance are identical. In a binomial sampling distribution, this condition is approximated as p becomes very small, providing that n is relatively large. The mean and variance of a binomial sampling distribution are equal to np and npq, respectively (with q=1p). As p approaches zero, the value of npq approaches that of np, and the binomial distribution accordingly approximates the form and properties of the Poisson.
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