Casner says. "If you're sitting there watching the system and it's doing
great, it's very tiring." In fact, it's extremely difficult for humans to
accurately monitor a repetitive process for long periods of time.
This so-called "vigilance decrement" was first identified and measured
in 1948 by psychologist Robert Mackworth, who asked British radar operators
to spend two hours watching for errors in the sweep of a rigged analog clock.
Mackworth found that the radar operators' accuracy plummeted after 30 minutes
; more recent versions of the experiment have documented similar vigilance
decrements after just 15 minutes.