Calculating Percentage Change in Changing Times

Welcome back to The Public Library Blueprints! After a short hiatus in April, this post tackles a topic that can trip up even practiced researchers. Finding the percentage change between two numbers can be helpful if your data consists of two related numbers, where one is an original or past value and the other is the new or updated number. It is also useful for comparing changes between two sets of numbers without being distracted by their actual values. While calculating percentage change can be done by hand with a short formula, and there are many online calculators that will do the math for you, there are some important principles to bear in mind that will help you ensure percentage change is communicated accurately and effectively.

We encounter percentage changes fairly often in everyday life. Sales are one of the first examples that come to mind. If you find the summer sandals you’ve been looking for on sale for 30% off, that sounds like a great deal, but it’s still helpful to know what the final price of the sandals will be. In this case, you’ve been given the percentage change already (a 30% decrease in price), and you are looking for the final price after this discount is applied to the original price. When calculating percentage change to analyze and present data you likely have the original and new numbers already and want to find the percentage change between them. Just as you might do if, for example, a friend tells you that their sandals were originally $40 dollars but they got them for $26, and now you’re curious who found the better sale. Your friend wins this one, because their sandals were 35% off. Here’s how to find out:

(Final Value – Initial Value) ÷ | Initial Value | × 100 = PERCENTAGE CHANGE

(26 – 40) ÷ | 40 | × 100 = -35

Returning to research, let’s talk about how, why, and when to incorporate percentage change into your data analysis. Assuming your data set already contains the original and new numbers, calculating the percentage change between these numbers can tell you and your audience more about their relationship. As with all Public Library Blueprint posts, we’re turning to the Public Library Annual Report (PLAR) data to learn more…

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