We may not always know the precise probability of en event occuring and instead need to estimate this. For dexample, if we are not sure whether a coin is faor, we might toss it many times and then use the proportion of times it lands heads as an probability estimate. Note that this estimate is itself usually a stochastic value as it is based on sampling. Often the frequncy estimate often has a binomial distribution, so that if the true probability is p and we use N tries to estimate it, then the proportion we measure has mean p and variance p(1-p)/N. However, this only applies when the estimate is based on lots of independent trials. In some circimstances more complex ways are needed to produce a probability estiamte.
Used in Chap. 19: page 329