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(also known as: fishing for data)

Description: Drawing a conclusion from correlations observed in a given sample, but only after the sample has already been drawn, and without declaring in advance what correlations the experimenter was expecting to find.  This is related to the multiple comparisons fallacy.

Example #1:

In looking at the records of my students, I have found that 9 out of 10 are an only child.  Therefore, society is moving towards one-child families.

Explanation: When you start looking at data with no expectations, anything goes, and any data due to random, statistical anomalies will stand out as “odd”.  In this case, the fact that 9 out of 10 kids don’t have siblings is outside of the norm, but that is the nature of probability.  If you were hypothesizing that most kids don’t have siblings, and you found this data, then it would provide more of a reason to do further research in making a more justified conclusion. 

Example #2:

In looking at the difference between 100 Christians and 100 atheists, we found that Christians were significantly more likely to eat tuna fish.

Explanation: When you fish for data, you are bound to catch something -- in this case tuna.  Notice that because we were looking for anything, we are bound to find it.

Exception: At times, truth is revealed in data whether we look for it or not, but we need to realize that meaningless statistical anomalies are to be expected when looking at data.

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