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Jaccard Index

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Jaccard index measures the commonality between two sets. Its value always lies between 0 and 1. A higher value of the Jaccard index means that there is a greater similarity between the two sets.

Given two sets A and B, the formula for the Jaccard Index is,

Formula for Jaccard Index
Formula for Jaccard Index

How to calculate the Jaccard index?

  1. Given two sets A and B, count the number of point which lie in both sets.
  2. Count the total number of points which lie in either of the two sets.
  3. Divide the two numbers as shown in above formula to obtain the value of the index.
  4. Multiply the result by 100% if you want to express the similarity in the form of a percentage.

Interpretation of the Jaccard Index:

If the Jaccard index is closer to 1, it means that the two sets have more similarity. If the index is closer to o it means the two sets are dissimilar. The Jaccard index being equal to 0 means that the two sets are completely disjoint.

Example: We are given two sets of people – those who like to drink tea and those who like to drink coffee (the sets being denoted as A and B respectively). If 30 people like tea and 40 people like coffee and 10 people like both. Find the Jaccard index for the two sets.

Solution: Given, n(A)=30 , n(B)=40 and n(A∩B)=10

By the addition rule, n(AUB)= n(A)+ n(B)- n(A∩B) = 30+40-10 = 60

Substituting all this in the above formula we get,

J(A,B) = n(A∩B)/ n(AUB) = 10/60 = 0.1667 or 16.67%

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