Availability and Representativeness

Psy315h - Class 26

(This page last updated 25 November, 2005.)

Induction: Starting with specific facts and drawing general conclusions from them.

Deduction: Starting with a general principle and working out specific facts that follow from it.

On this page, we will examine two different specific problems of induction and how we solve them.

Strategy for estimating the frequency with which something occurs.
Try to recall as many instances as possible. The more instances recalled, the higher the estimate of frequency.
Kahneman and Tversky propose that it is a general strategy used in many different situations.
In most real-world situations in which it is used, Availability usually works well.
However, it is not guaranteed to give the right answer.
It is a heuristic, a quick procedure that produces answers that are usually right, but not always.

Subjects see list of names. Then asked whether list contained more men or women.
Some names are famous, and some are not.
All lists have equal numbers of men and women. If the men on a list were relatively more famous, subjects report that the list has more men.
If women on list were relatively more famous, subjects report that the list has more women.

Recent events are easier to remember than earlier events.
If Availability is used to estimate the frequency of an occurrence, then something that has occurred recently will seem to occur more frequently.
This explains:
- why people are more likely to fasten seatbelts after seeing an accident.
- why you might be more likely to back up your computer files after a friend loses theirs.

In some cases, Availability can lead to conclusions that violate the conjunction rule.

Another heuristic proposed by Kahneman and Tversky.
Estimate probability that an exemplar belongs in a category by assessing how representative that event is of the appropriate category.
In the example in class, Linda is more representative of a feminist than a bank teller.
Therefore, descriptions including feminism seem more likely than "bank teller" by itself.
In this case, representativeness leads to a violation of the conjunction rule.
- P(A&B) <= P(A)
- The probability of (bank teller and feminist) will always be less than or equal to the probability of (bank teller).

Base Rate: How often an item of a particular type appears in the population.
People generally ignore base rates in categorizing things. They tend to rely more on representativeness.
Example in text, p. 393.

next class: Deduction

Psych 315H: Cognitive Psychology Kyle Cave Psychology Dept. U. Mass.