Paired line plots (a.k.a. “reaction norms”) to visualize Likert data

In the past year, I’ve been working on several projects that used Likert-scale data (e.g., 1 = strongly disagree, 5 = strongly agree). And, in several instances, there were questions that it made sense to pair. As one example (which I blogged about in more detail earlier this month), for Morgan Rondinelli’s undergraduate thesis project on student mental health, we asked students whether they would think less of someone who sought mental health care and also whether they thought others would think less of someone who sought mental health care? In that case, I was curious not just about the aggregate percentages in the different categories, but also how individual views compared. So, being a good evolutionary ecologist raised on reaction norms (where genotypes are plotted in different environments, with the points for each environment connected by a line), I made a paired line plot:

Note: the individual lines are gray and slightly transparent, so more common pairs of responses appear darker and thicker

This figure shows me that no students viewed themselves as more judgmental than the average: none of the lines go up. That’s not information that I could get from other ways of plotting the data (shown in my earlier post).

A different example comes from a project studying student views on climate change, which I’m working on with Susan Cheng and JW Hammond. We asked students the same questions at the beginning and end of the semester. To focus on one question, we asked students “Do you think climate change is happening” at the beginning of the semester and again at the end of the semester. The overall results were promising:

 

My take on these results is: nearly all students come in to the course accepting that climate change is happening (good news!) and they become more sure of that over the course of the semester (also good!)

But, again, I was curious about how individual students changed. So, again, I went to a paired line/reaction norm-style plot:

Note: the individual lines are gray and slightly transparent, so more common pairs of responses appear darker and thicker

Some students became more skeptical/less sure about climate change over the course of the semester, but the vast majority either stayed the same or became more confident/more sure. That’s good news, too!

I showed these plots to several people – collaborators on the projects, folks who do pedagogy research, colleagues at Michigan’s Office of Academic Innovation. Their initial reaction was generally a furrowed brow, tilted head, or something along those lines. That was my clue that this is perhaps not a typical way of plotting Likert scale data! However, after a minute or so, they generally seemed to think it was an interesting way of viewing the data.

This led me to wonder if I had accidentally stumbled onto a useful, not widespread way of plotting Likert scale data, by virtue of my training in evolutionary ecology. I’m not sure – I definitely haven’t read enough literature in fields that typically use Likert-scale data to know for sure. But my google image search only found two papers that seems to have paired line plots for Likert scale data. (I probably spent more time on this than I should have, but it was far from exhaustive and definitely sensitive to keywords; it’s entirely possible this is common and I just don’t realize it!)

So, just in case this is useful for others working with Likert scale data, I figured I’d put the idea out there that paired line charts can be a useful way of getting a sense of how individual views compare for paired questions. In the future, I’m planning on comparing these with heat maps, but R and I were not getting along well on Friday. Hopefully there will be a future post with that comparison!