Numbers can bewilder our hunter-gatherer brains. For more than 95 percent of human history, folks were not processing written numbers or words. But they were processing visual information in the form of color, shape, and size. It’s not surprising that our brains, evolved over many thousands of years, are better at understanding data in visual form than in word and number form. So when numbers confuse, try “translating” them to the visual.
Here’s a great example of a number that makes me scratch my head: “54% more students with monitors improved attendance than students without monitors.” The statement relates to a fictional program that (like some non-fictional programs) pairs students with monitors to boost their attendance. At first blush, to me, that sounds pretty impressive. It sounds like this: if 10% of the students without monitors improved their attendance, then 64% (10% + 54%) with monitors improved their attendance. Or, put another way, six times as many kids with monitors improved their attendance as kids without monitors.
But my brain just made a wrong turn. That 54% is showing what statisticians call “relative difference.” And the problem with this type of stat is that indicators with low values have a tendency to produce large relative differences even when the “absolute difference” is small.
Okay, still bewildered? No worries, I give you now a picture for your primitive brain. Let’s say, in our fictional program, there are 10 students per class. In one class, all of the kids got paired with monitors. In the other class, none of the kids did. The picture below shows how many kids in each class improved their attendance.
So the difference (aka “absolute difference”) is 1.4 (4.0-2.6) which means that 1.4 more kids in the class with monitors improved their attendance. How did that measly 1.4 become 54%? Well, relative difference is calculated as the absolute difference divided by the “standard” which, in this case, is the class without monitors. So 4.0 minus 2.6 divided by 2.6 or .54, which when expressed as a percentage is 54%.
If relative difference requires varsity level processing for many of us, then percentages are junior varsity. So if I were visualizing the difference between the two groups, I would stay away from both and use an icon chart, like the one above. I might make it even more concrete by showing 25 person icons in each group since the typical elementary school classroom has 25 students. I would then use color to show that 6.5 students out of 25 without monitors had improved attendance and 10 students out of 25 with monitors had improved attendance. So, if you bring the program to a typical classroom, you might expect it to improve the attendance of an additional 3 to 4 kids.
Bottom line? Numbers can be like road signs pointing us in the wrong direction. To move folks in the right direction, make your message concrete and visible.
See other data tips in this series for more information on how to effectively visualize and make good use of your organization's data.