attention

You are blind to the obvious when focused elsewhere.

A 1999 study showed that 46% of the participants failed to see an unexpected object while tracking moving items.

Change 'half' to '46%'

You see what you look for, not what is there.

Where this comes from

Study: Simons and Chabris, 1999
What the check found: The original study (Simons and Chabris, 1999) observed a 46% failure rate. 'Half' is an imprecise approximation.
Confidence: Well replicated

One of these every day, and every one names the study it came from. When a finding is contested, it says so.