How to Use Excel for Binomial Probability

by C. Taylor

Binomial probability calculates the chance of certain events happening, which have only two possible outcomes, such as flipping a coin. Excel offers a function that can calculate these sometimes complex calculations easily. When calculating binomial probabilities, you can achieve three basic calculations: The probability of exactly some number of successes in a given number of trials, the probability of at most some number of successes, and the probability of at least some number of successes.

1

Right-click the cell where you will be performing the calculation, and select "Format Cells." Click "Percentage" from the "Number" tab, and click "OK" to make the resulting calculation a percentage, rather than a standard decimal number.

2

Enter "=BINOMDIST(successes,trials,probability,cumulative)" in the cell, replacing the parenthesized names with the actual data. Replace "successes" with the number of successes for which you wish to calculate the probability. Replace "trials" with the number of attempts. Replace "probability" with the known probability of a single success in a single trial. Replace "cumulative" with either "true" or "false", depending on if you want to calculate "at most" or "exactly" some number of successes, respectively. As an example, to calculate the probability of achieving exactly 12 heads out of 20 coin flips, which have a 50 percent

probability, you would enter "=BINOMDIST(12,20,0.5,false)". To calculate the probability of achieving zero to 12 heads out of 20 coin tosses, you would enter "=BINOMDIST(12,20,0.5,true)".

3

Enter "=1-BINOMDIST(successes-1,trials,probability,cumulative)" to calculate the probability of achieving at least some number of successes. As an example, to calculate the probability of achieving 12 to 20 heads out of 20 coin flips, you would enter "=1-BINOMDIST(11,20,0.5,true)". This effectively calculates the chance of achieving no more than 11 heads out of 20 coin flips, but then subtracts it from one, which results in the probability of 12 to 20 heads out of 20 flips.

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