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logic: biased or suppressed data

These errors in logic both involve the misuse of data. A writer who resorts to biased data collects evidence from sources on only one side of the argument. A writer who suppresses data chooses data selectively, ignoring evidence that doesn’t support the conclusion he or she wants.

Example of biased data:

"More than 85% of people surveyed believe that the summer break for secondary school students is too short. Therefore, the academic year should be reduced to allow for a longer vacation."

[The people surveyed were all secondary school students. We can predict that most of them would prefer a longer vacation! Therefore, they represent only one side of the argument, and the sample is biased.]

Example of suppressed data:

"This new product has passed several tests. It meets safety standards and is ready to go on the market."

[The argument sounds good on the surface–but in this case, the writer has focused on positive test results only and has ignored a small number of tests showing evidence of risks. The writer has therefore suppressed important data.]

In building a logical argument, it is important to be sure that all the facts from both sides of the argument have been fairly presented.