In Canada, a radar operator watching for Soviet missiles stared at his screen in disbelief. Hundreds of objects had suddenly appeared over the North Pole, headed for the US. He picked up the telephone, and made an urgent call...

Welcome to the world of signal detection!

Although the eyes collect graded information about the world, we often need to make binary yes-or-no decisions about what we see. Is that waving thing behind the big tree the frond of a plant or a hungry tiger? Signal detection theory was developed to help us understand how a continuous and ambiguous signal can lead to a binary yes/no decision. Signal detection theory is useful in understanding automatic devices, like motion detectors that open grocery-store doors, and our own senses.