forecastingprinciples.com Reviews of Important Papers on Forecasting
Before 1985 Reviews
Review of:

M. Manis, I. Dovalina, N. E. Avis, and S. Cardoze (1980), "Base rates can affect individual predictions," Journal of Personality and Social Psychology, 63, 231-248.

M. Bar-Hillel and B. Fischhoff (1981), "When do base rates affect predictions?" Journal of Personality and Social Psychology, 4, 671-680.

M. Manis, N. E. Avis, and S. Cardoze (1981), "reply to Bar-Hillel and Fischhoff," Journal of Personality and Social Psychology, 41 681-683.


Judgmental forecasters should consider what typically happens in a situation (the 'base rate') as well as specific information available about the case in hand. Although the specific information should be considered only if it is valid and reliable, earlier research showed that even irrelevant specific information led people to ignore the base rate. Manis et al (1980) conducted four experiments and re-examined previous studies. From this they defined a set of conditions in which judges showed much sensitivity to base rates. Their study seemed convincing to me until I read the Bar-Hillel and Fischhoff paper, which reinterpreted Manis et al. and concluded that their results were consistent with previous research; base rates are important when the subject does not receive information on representativeness (evidence that the subject of the prediction fits a stereotype. Manis et al. (1981) is a well-reasoned reply to Bar-Hillel and Fischhoff. The combination of articles helps to specify the conditions under which forecasters should not trust their intuitions when interpreting base rates plus specific information. An interesting finding is Manis (1980) was that subjects did not seem to be aware when they used base rates in their predictions. I predict we'll hear more on the subject of base rates.