Calculating binomial probabilities using a calculator can get very tedious very quickly. This is especially true when calculating cumulative probabilities. For example, suppose we are testing a sample of 50 parts (with replacement) where we expect an average defect rate of 1% and we want to know what the probability is of finding 2 or fewer defects in the batch. This is a cumulative probability, so we have:

Which means we have to calculate:

which provides many chances to make errors.

Fortunately, we can use the Excel function **BINOMDIST**.
From the __I__nsert menu, select __F__unction, then select **BINOMDIST **from
the statistical functions, which brings up the
following window:

The entries are:

Number_s - the number of "successes" in the trial (2 in our problem)

Trials - the number of trials in our experiment (50 in our problem)

Probability_s - the probability of a success in a trial (0.01 in our problem)

Cumulative - Is the probability to be calculated cumulative (true in our problem) or not (false)

The **BINOMDIST** function can be entered directly. For
our problem we would enter:

=binomdist(2,50,0.01, true)