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[M]any of the most compelling usability test insights come not from the elements that are evaluated, but rather those not evaluated. They come from the almost unnoticeable moments when a user frowns at a button label, or obviously rates a task flow as easier than it appeared during completion, or claims to understand a concept while simultaneously misdefining it. The unintended conclusions—the peripheral insights—are often what feed a designer’s instincts most. Over time, testing sessions can strengthen a designer’s intuition so that she can spot troublesome design details with just a glance. Simply put, usability tests can provide huge insight into the patterns and nuances of human behavior.
From The Myth of Usability Testing by Robert Hoekman Jr.