I sat behind the glass watching someone use the app, and she said she loved the new layout. But her hands told a different story, she hunted around for that button for a full minute. I can run the test now, I just cannot always tell which one to believe, her words or her fumbling.
More people experience this than they realize.
You can run tests but struggle to separate what users say from what they actually do.
If this sounds familiar, the Library can help you find the bigger picture.