Greetings from Bustins Island, Maine, where every summer I mix a batch of watermelon mojitos, climb into the hammock, and reflect on the year to date, the work ahead, and the Sox’ standing in the ALE (as of 8/25, first place – 5 games ahead of the Yankees).
On the professional front, I am heartened by the fact that data is increasingly being analyzed and used to improve our health and systems of care, and by growing awareness of the best practices of data visualization and visual intelligence. There is all the same a lot of work to be done!
The good news I’m hearing from clients is they’ve improved their performance on many quality measures and are consistently meeting their goals. What remains to master is how to monitor their results using something other than a bar chart.
Though useful and easy to interpret most of the time, bar charts can make it difficult to see very small differences between multiple measures on a dashboard or report; and they can take up a lot of space better used to display additional important metrics. Clients often ask, pleadingly, if “just this once” they can ignore the best practice of starting a bar graph at zero. My inner Dana Carvey kicks in with “…wouldn’t be prudent …not gonna do it” (yes, it sounds funnier when he says it). Won’t be auditioning for the host slot on SNL anytime soon, though, even with help from those mojitos; so back to visualizations.
As you can see in this first display, the points showing actual performance for all four measures versus their respective targets appear quite closely grouped, so it’s hard to tell them apart. This hindrance in turn tempts us to break the best-practice rule of starting a bar graph at zero, and instead start it at a higher value. To remind ourselves why this is a bad idea, let’s review that rule.
Bars display and compare the size of different values; if they begin at a value greater than zero, the true size of what they are designed to measure is distorted. Starting the scale at a value > 0 and decreasing the increments across the chart or down the bars (that is, along the X or the Y axis) to magnify the values can make the differences displayed seem much larger than they actually are. (Take a look at my earlier post on this topic here. You may decide to eat less oatmeal after you do).
This illustration shows the effect of interval distortion.
Consider these options for resolving this conflict between visualization guidelines and the need for a quick fix.
Points and Lines
Using points and lines to display values like these can be a good alternative to a bar chart because that frees us from having start the scale at zero. The points and lines format also allows us to display viewer-required details in far less space on a dashboard or report than does a bar graph.
One of my favorite ways to display performance versus target data on a dashboard or report is with a deviation graph, which illustrates the relative difference between two values. Though each performance metric may have a different target figure, this graph shows only the distance from target for each, so we can line them all up one after the other (see below). A deviation graph is neat, clear, and space-saving. (Two more articles on this useful visualization appear here and here.)
Small Multiple Line Graphs
Depending on viewer requirements, you might also consider displaying the kind of data we’ve been discussing in small multiple line graphs. Unlike bar graphs, line graphs do not have to begin at zero, because the line isn’t comparing the sizes of two values. Instead, lines display values changing or trending over time.
If for example you need to show how your organization has performed over time compared to each measure’s unique target, you can create separate graphs for each, careful to make the scales begin and end with the same values, then arrange them in sequence. This technique makes it easy to see how the measures have changed, and compare measures both to targets, and to each other. (Two previous small multiples posts are here and here.)
In the past decade or so, the health and healthcare industries (among others) have increased their awareness of the need to discover and embrace best practices of caring for patients and delivering services. This has also happened with data visualization.
As we become more familiar and comfortable with the science behind and research on visual intelligence and cognition, we too must embrace these best practices to ensure that our visualizations are correctly displaying the data, that our message is clear, and that people are informed, inspired, and above all moved to action to improve the healthcare we help deliver.
Speaking of caring for people, the hammock is beckoning. I need a nap before tonight’s game. Life is good.
P.S. For those who ask, “What about wide variation data?” – a different, albeit related subject – I call your attention to a not-so-recent post I wrote, here.