“Standard deviation” sounds like an oxymoron. Any high school student knows that you can’t be both standard and a deviant at the same time. And any high school stats class will clarify, in the first week or so, what standard deviations are really about. And most high school students will hold that knowledge for a semester and then delete it to make space for more valuable knowledge.
But I come to you today to re-enter the standard deviation onto your personal hard drive. Because it is valuable even to those of us who are not statisticians or researchers. It’s a great tool for anyone trying to understand what is going in organizations trying to up their impact.
60-Second Data Tip #13 addressed what averages obscure. The answer was: how spread out your data points are around the average. The standard deviation tells you just how spread out they are. You might think of the average as the “standard” (the person at your high school who was average in every way). And you might think of the rest of the data points as “deviants” with some deviating from average just a bit (perhaps a kid with a nose ring who was otherwise sporty) and others deviating a lot (full-on Goth).
Here’s a more nonprofity example. If the average wage of participants in a job-training program is $19 per hour, this might obscure the fact that a few participants are earning over $30 per hour and the majority are earning below $10 per hour.
A standard deviation close to 0 indicates that the data points tend to be very close to the mean. As standard deviation values climb, data point values are farther away from the mean, on average. So a job-training program aiming for an average wage of $17 per hour among participants might want to see a pretty low standard deviation in wages to feel confident that the large majority has reached the goal.
I won’t scare you with the formula for calculating the standard deviation. You can keep that off your hard drive. Any spreadsheet program will calculate it for you. For example, for data in rows 1-350 in column A on an Excel spreadsheet, just use enter “=STDEV(A1:A350)” to get it.
See other data tips in this series for more information on how to effectively visualize and make good use of your organization's data.