Base Rate Fallacy

(also known as: neglecting base rates, base rate neglect, base rate bias [form of], prosecutor's fallacy [form of])

Description: Ignoring statistical information in favor of using irrelevant information, that one incorrectly believes to be relevant, to make a judgment.  This usually stems from the irrational belief that statistics don’t apply in a situation, for one reason or another when, in fact, they do.

Example #1:

Only 6% of applicants make it into this school, but my son is brilliant!  They are certainly going to accept him!

Explanation: Statistically speaking, there is a 6% chance they will accept him.  The school is for brilliant kids, so the fact that her son is brilliant is a necessary condition to be part of the 6% who do make it.

Example #2:

Faith healing works, but not all the time, especially when one’s faith is not strong enough (as generally indicated by the size of one’s financial offering).  Unbiased, empirical tests, demonstrate that a small but noticeable percentage of people are cured of “incurable” diseases such as cancer.

Explanation: This is true.  However, what is not mentioned in the above is the number of cases of cancer that just go away without any kind of faith healing, in other words, the base rate of cancer remission.  It is a statistical necessity that among those with cancer, there will be a percentage with spontaneous remission.  If that percentage is the same as the faith-healing group, then that is what is to be expected, and no magic or divine healing is taking place.  The following is from the American Cancer Society:

Available scientific evidence does not support claims that faith healing can cure cancer or any other disease. Some scientists suggest that the number of people who attribute their cure to faith healing is lower than the number predicted by calculations based on the historical percentage of spontaneous remissions seen among people with cancer. However, faith healing may promote peace of mind, reduce stress, relieve pain and anxiety, and strengthen the will to live.[1]

Exception: If there are factors that increase one’s odds and alter the known statistical probabilities, it would be a reasonable assumption, as long as the variations from the statistical norm are inline with the factors that cause the variation.  In other words, perhaps the mother in our first example knows that her son is gifted musically, that counts for something, then it is not unreasonable to expect a better than 6% probability -- but assuming a 50%, 80%, or 100% probability, is still committing the fallacy.

Tip: Take some time in your life to read a book or take a course on probability.  Probability affects our lives in so many ways that having a good understanding of it will continually pay off.

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