Subadditivity effect
Encyclopedia
The subadditivity effect is the tendency to judge probability of the whole to be less than the probabilities of the parts.
For instance, subjects in one experiment judged the probability of death from cancer in the United States was 18%, the probability from heart attack was 22%, and the probability of death from "other natural causes" was 33%. Other participants judged the probability of death from a natural cause was 58%. Natural causes are made up of precisely cancer, heart attack, and "other natural causes," however, the sum of the latter three probabilities was 73%, and not 58%. According to Tversky and Koehler (1994) this kind of result is observed consistently.
The same mechanisms may underlie an effect of familiarity on probability judgment. More familiar events are more available. This is known as the availability heuristic
. We find it easier to think of reasons why these events will and will not happen. In an experiment carried out by Fox and Levav (2000), they asked students at Duke University
which of two events was more likely to occur. The first one was "Duke men's basketball
defeats UNC men's basketball
at Duke's Cameron Indoor Stadium
in January 1999," and the other one was "Duke men's fencing defeats UNC men's fencing
at Duke's Cameron Card Gym in January 1999." The researchers postulated that since Duke students are much more familiar with basketball than with fencing, 75% of the students thought the basketball victory was more likely. Other students answered exactly the same questions, however, with Duke and UNC switched around. After the change, 44% of the students said that a UNC victory in basketball was more likely than a UNC victory in fencing. 44% plus 75% is 119%, which is larger than 100%, and only one such basketball game would be played. In this experiment, familiarity with basketball led subjects to think of the basketball event as more likely than the fencing event, regardless which basketball event was described.
For instance, subjects in one experiment judged the probability of death from cancer in the United States was 18%, the probability from heart attack was 22%, and the probability of death from "other natural causes" was 33%. Other participants judged the probability of death from a natural cause was 58%. Natural causes are made up of precisely cancer, heart attack, and "other natural causes," however, the sum of the latter three probabilities was 73%, and not 58%. According to Tversky and Koehler (1994) this kind of result is observed consistently.
The same mechanisms may underlie an effect of familiarity on probability judgment. More familiar events are more available. This is known as the availability heuristic
Availability heuristic
The availability heuristic is a phenomenon in which people predict the frequency of an event, or a proportion within a population, based on how easily an example can be brought to mind....
. We find it easier to think of reasons why these events will and will not happen. In an experiment carried out by Fox and Levav (2000), they asked students at Duke University
Duke University
Duke University is a private research university located in Durham, North Carolina, United States. Founded by Methodists and Quakers in the present day town of Trinity in 1838, the school moved to Durham in 1892. In 1924, tobacco industrialist James B...
which of two events was more likely to occur. The first one was "Duke men's basketball
Duke Blue Devils men's basketball
The Duke Blue Devils men's basketball team is the college basketball program representing Duke University in the Atlantic Coast Conference of NCAA Division I...
defeats UNC men's basketball
North Carolina Tar Heels men's basketball
The North Carolina Tar Heels men's basketball program is the intercollegiate men's basketball of the University of North Carolina at Chapel Hill, and is considered one of the most successful programs in NCAA history...
at Duke's Cameron Indoor Stadium
Cameron Indoor Stadium
Cameron Indoor Stadium is an indoor arena located on the West Campus of Duke University in Durham, North Carolina. It is home to the Duke Blue Devils men's and women's basketball teams, the women's volleyball team, and the men's wrestling team....
in January 1999," and the other one was "Duke men's fencing defeats UNC men's fencing
Fencing
Fencing, which is also known as modern fencing to distinguish it from historical fencing, is a family of combat sports using bladed weapons.Fencing is one of four sports which have been featured at every one of the modern Olympic Games...
at Duke's Cameron Card Gym in January 1999." The researchers postulated that since Duke students are much more familiar with basketball than with fencing, 75% of the students thought the basketball victory was more likely. Other students answered exactly the same questions, however, with Duke and UNC switched around. After the change, 44% of the students said that a UNC victory in basketball was more likely than a UNC victory in fencing. 44% plus 75% is 119%, which is larger than 100%, and only one such basketball game would be played. In this experiment, familiarity with basketball led subjects to think of the basketball event as more likely than the fencing event, regardless which basketball event was described.