Trend Lines Can Tell Lies
Don't be Mislead
I was reading an old issue of the Joint Commission Journal on Quality and Patient Safety. In an article about patient falls, the authors used a run chart with a trend line to assert that they had achieved a 9.9 percent reduction in patient falls.
Trend Lines Aren't Trends
The trend line suggests a trend, but does one exist? If we use an XmR control chart to plot the same data, we find that the process is stable and unchanged:
The trend line has lied about the improvement. This is why primitive line charts, run charts and trend lines can be so misleading--they aren't statistically useful. You need a control chart to detect actual changes in performance.
Over a decade ago I worked with a hospital that actually reduced falls and you could see it in their data. There's a real downward trend of six consecutive points in a row:
While the falls average is higher than the first example, in this case, we've actually demonstrated a sustained improvement.
Don't Let Primitive Charts Like Run Charts and Line Graphs Spoil Your Analysis
Learn how to use control charts to analyze data and demonstrate real results.
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