Two-alternative forced choice

Two-alternative forced choice (or 2AFC) is a way of testing someone's ability to tell the difference between two different choices. For example, you might be shown a picture of two different shapes and asked to choose the one that looks more like a square. Or, you could be shown two different images of nature scenes and asked which one looks better. In this way, 2AFC is a way of measuring how well you can tell the difference between two different things.