Probabilities turned to frequencies

Previously I mentioned the fact that when making decisions we use representative heuristic which is also known as similarities which in turn gives room for error to what is know as the base rate fallacy or neglect. Base rate fallacy is when we do not take into account the proportion of people who are like the description given but are actually something else. Gigerenzer, 1994, proposed that when probability questions are turned to frequencies such errors are less likely to occur. 


A probability question  is like: this taken from Gigerenzer,1994 article.
"1. A device has been invented for screening a population for a disease known as psylicrapitis. 2. The device is a very good one, but not perfect. 3. If someone is a sufferer, there is a 90% chance that he will be recorded positively. 4. If he is not a sufferer, there is still a 1% chance that he will be recorded positively. 5. Roughly 1% of the population has the disease. 6. Mr. Smith has been tested, and the result is positive. The chance that he is in fact a sufferer is:______.


What was found here was that most participants gave a response of 85% which is close to the 90% hit rate given in the example despite the 1% chance. Making such judgemental error is what is known as the base-rate fallacy. However when the question was switched to frequency readers could digest the question more easily. The frequency question is as follows:


"Out of every 100 persons tested, we expect 1 to have the disease; and the device is nearly certain to say that he has. Also, out of that 100, we expect the machine to say that 1 healthy person has the disease. Thus, in the long run, out of every 100 persons tested, we expect 2 positive results, one of which will be correct and the other incorrect. Therefore the odds on any positive result being valid are roughly even".
Response to this frequency format was 50-50.


It is not to say that such errors do disappear when probabilities are turned to frequencies but peoples are answer are usually better in frequency questions than probability questions. The likelihood of making such judgements are reduced are less likely.


References

Gigerenzer, G. (1994). Why the distinction between single-event probabilities and frequencies is important for psychology (and vice versa). In G. Wright and P. Ayton (Eds.), Subjective probability. Chichester, UK: Wiley.