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    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?
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    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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