How to Calculate Binomial Probabilities
- 1). Define "success". The usual language of binomial probabilities classifies results as "successes" and "failures" and calls events "trials". A success could be a head resulting on a flip of a coin.
- 2). Open Excel and enter the number of successes in a cell. For example, if we get 25 heads we could enter 25 in cell A1.
- 3). Enter the number of trials in another cell. For example, if we had tossed the coin 40 times, we could enter 40 in cell A2.
- 4). Decide on a probability and enter it in a cell. In our example, we might suppose that the probability of heads was .5, so we could enter that in cell A3.
- 5). Calculate the probability using the BINOMDIST function in Excel. In our example, go to cell A4 and enter =BINOMDIST(A1,A2,A3,FALSE) for the probability of exactly 25 heads in 40 tosses, or =BINOMDIST(A1,A2,A3,TRUE) for the probability of 25 or fewer heads in 40 tosses. For the probability of more than 25 heads, subtract the latter value from 1.
