Signal detection theory - part 1 | Processing the Environment | MCAT | Khan Academy

Summary

Signal Detection Theory (SDT) explains how people make decisions when sensory information is uncertain. It examines how strong a signal must be before someone detects it and how a person’s decision strategy affects their responses.

The lesson illustrates SDT with a bright green circle, which is easy to notice, and a faint green dot, which is harder to detect. The theory originated in radar detection, where operators had to decide whether an ambiguous signal came from a ship, a whale, or a school of fish. It also applies to psychological tasks, such as deciding whether a word in a second list appeared in an earlier list, and everyday situations, such as judging whether a traffic light is green through fog.

The Four Possible Outcomes

A decision depends on whether the signal is actually present and whether the observer says it is present:

• Hit: The signal is present, and the observer says it is present.
• Miss: The signal is present, but the observer says it is absent.
• False alarm: The signal is absent, but the observer says it is present.
• Correct rejection: The signal is absent, and the observer says it is absent.

A strong, clear signal is generally easier to detect and tends to produce more hits. A weak or faint signal is harder to distinguish, making misses more likely.

Two Important Variables

• d′ (D-prime): The strength or detectability of the signal. Stronger signals are easier to distinguish from their absence.
• c: The observer’s decision strategy, or how much evidence they require before saying a signal is present.

Decision Strategies

• Conservative strategy: Say “present” only when very certain. This can reduce false alarms but may lead to more misses.
• Liberal strategy: Say “present” with less evidence. This can increase hits but may also produce more false alarms.

Note: The subtitles briefly describe a “yes” response to a signal that is actually present as a false alarm. Under the outcome definitions given in the lesson, that response is a hit. A false alarm occurs when the signal is absent but the observer says it is present.

Speaker and Source

The voiceover is by an unnamed Khan Academy instructor, who explains the concepts.