For example, the composition of women 18-34 on the imaginary site QRS.com is 12%. Let’s say that women 18-34 represent 10% of the
U.S. population. The composition index of women 18-34 on QRS.com is 120, derived by dividing 12% by 10%.

The composition index (or, simply index) of 120 means that the QRS.com site has a composition of women 18-34 that is 20% higher than the composition of women 18-34 in the U.S. population.

If the index had been 100, it would mean that the composition of women 18-34 on the QRS.com site is the same as the composition in the U.S. population.