Simpsons Diversity Index

This is a calculation done by ecologists that is a measure of diversity which takes into account both richness and evenness of species. The term ‘Simpson’s Diversity Index’ can actually refer to any one of a range of closely related indices (Simpson’s Diversity Indices), but QCAA specifically deals with the Simpson’s Index (D).

Simpson’s Index (D) measures the probability that two individuals randomly selected from a sample will belong to the same species.

This is the version of the equation in the QCAA syllabus:

D=1n(n1)N(N1)

= sum of

n = number of individuals

N = total number of individuals

The value of this index also ranges between 0 and 1. The greater the value, the greater the sample diversity. You may see this equations in exactly the same form, just omitting the 1 -. With this index, 0 represents infinite diversity and 1, no diversity. This is neither intuitive nor logical, so that’s where the 1 – D comes from.

The following pages will provide step-by-step instructions to show you how to calculate Simpson’s Diversity Index.