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# S2 Binomial or Poisson Watch

1. In a question where I have been given the number of trials, the probability of success and hence the mean, Should I use the Binomial Distribution Formula or the Poisson distribution Formula?
2. In Edexcel, the rule is that if you are given the number of trials and the probability, you use Binomial distribution as long as n is below 50. If n is large (above 50) and p is small (below 0.05 I think) you can use Poisson APPROXIMATION. Poisson DISTRIBUTION is only ever used when the events are singular and occur at a constant rate and you're given the mean, not n or p. If n is large and close to 0.5 (or n is extremely large) you may use Normal approximation.

The difference between Approximation and distribution being that you would use a different distribution to give an approximate answer to scenario with a distribution that is difficult to calculate. Hope this clears things up.

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Updated: July 15, 2016
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