Darrell Huff

It is worth keeping in mind also that the dependability of a sample can be destroyed just as easily by invisible sources of bias as by these visible ones. That is, even if you can’t find a source of demonstrable bias, allow yourself some degree of skepticism about the results as long as there is a possibility of bias somewhere.

Darrell Huff, How to Lie with Statistics, New York, 1954, p. 19