I can sit someone down and show them my design and ask if they like it. But turning that into a real test, one that tells me something I did not already believe, ties me in knots. I end up asking leading questions and nodding along, and I walk away having learned nothing.
More people experience this than they realize.
You can show a design to someone, but running a test that actually teaches you baffles you.
If this sounds familiar, the Library can help you find the bigger picture.