To calculate the rate-of-change, we (1) divide the most recent data by the year-ago data, (2) multiply the result by 100 to generate a percentage, and (3) subtract 100 to get the growth or decline value.
For example, when we take (March 2023 data)/(March 2024 data), such as 56.7/53.8, for example, we get the proportion, or change in value. In this case, it is 1.054. Then, we take that number and multiply it by 100 to convert it to a percentage. We get 105.4%, which is the new value as a percentage of the old. Subtracting 100 from that number then gives us the growth or decline value. In this case, that value is +5.4%; the data we are analyzing increased by 5.4% over the course of 12 months. The 5.4% is the rate-of-change. Essentially, subtracting 100 "isolates" the growth/decline value of the percent change from new/old proportion.
For a more in-depth explanation, see our additional methodology resources.