If you want to find the number of three-letter words you can make using these five letters, you might consider that the duplicate “O”s do not form different words.įor instance, “ROT” and “ROT” using the different “O”s are the same word, so they would not be counted as separate permutations in this example. But in some cases, you may want to allow for the repetition of duplicate values.įor example, let’s say you have the letters “FOORT”. So far, the formulas to calculate permutations have not allowed any repetition in the sample, and the assumption has been that each element is unique. Thus the number of permutations of r items in a set of n items is equal to n factorial divided by n minus r factorial. The following formula defines the number of possible permutations of r items in a collection of n total items. Once you know the number of permutations of a set, you can calculate the probability of each one of them occurring. There is a formula to calculate the number of possible permutations of items in a set. The number of possible permutations for items in a set is often represented as nPr or k-permutations of n.Ī permutation is basically one possible way to represent a sample of items in a particular order from a large set. A permutation is a group of items from a larger set in a specific, linear order.

Permutation equation