Improvement Insights Blog
Line Charts with Trend Lines are Fake News
Line charts with trend lines are fake news. Here’s why:
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“I was at the Institute for Healthcare Improvement conference, and I go through and I look at all the post-improvement posters that are out there. I saw “Nurse Betty” (that’s what I’m going to call her) and she was standing in front of a poster and she was studying it. She was wearing a nice blue pantsuit [and] comfortable shoes because it’s a lot of walking in these conferences, but she was staring at it like crazy. She pulls out her cell phone she takes a picture of this improvement poster, and so I thought, “Well, I guess I better go find out what Nurse Betty was doing,” right? What was she looking at?
“I went over there and it turned out there was a line chart about reducing a certain kind of infection in a hospital (I think it was urinary tract infections or something). Fortunately, they put the numbers on there so I took a picture of it and I went back and actually put together a line chart. The chart on the poster had a trend line through it that looked like “Oh yeah, we’ve reduced infections, yay!” right? So I went ahead and I did it… but then unlike theirs, I put the trend line in but I added the goodness of fit metric called r squared. Now r squared was only 0.32 or 32 percent fit. Would you want to stake your reputation on a 32 percent fit? No! You know, once it gets over 50 it’s a little bit better, once it gets to 80 that’s a pretty good fit. I have seen one or two line graphs with trend lines there where the r squared (not shown on the chart, I might add) the r squared was over 80… sweet! I could trust that one.
“So what I want you to do is get this idea if you see a line graph with a trend line on it that’s fake news. You don’t know anything at all about that trend line; Excel will fit a trend line to anything. You know, I’ve seen trend lines where it was… you know, like six percent fit, but Excel will throw one on there for you. It doesn’t mean it’s good, it didn’t mean you got better, it might just mean that you faked yourself out.
“So next time you see a line chart with a trend line on it and no r squared value, you can just assume it’s fake news. It’s much easier to use a control chart, and then you can see if it actually had some sort of actual change.
“So that’s my Improvement Insight for this week: Line charts with trend lines are fake news. Same thing with bar charts, I might add. Start using control charts. They’ve been around for 100 years. Let’s all start using them.
“Let’s create a hassle-free America; hassle-free healthcare. Let’s go out and improve something this week. “