Explanation
Self-selection makes it difficult to determine causation. For example, one might note significantly higher test scores among those who participate in a test preparation course, and credit the course for the difference. However, due to self-selection, there are a number of differences between the people who chose to take the course and those who chose not to. Those who chose to take the course might have been more hard-working, studious, and dedicated than those who did not, and that difference in dedication may have affected the test scores between the two groups. If that was the case, then it is not meaningful to simply compare the two sets of scores. Due to self-selection, there were other factors affecting the scores than merely the course itself.
Self-selection bias causes problems for research about programs or products. In particular, self-selection makes it difficult to evaluate programs, to determine whether the program has some effect, and makes it difficult to do market research.
Read more about this topic: Self-selection Bias
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“My companion assumes to know my mood and habit of thought, and we go on from explanation to explanation, until all is said that words can, and we leave matters just as they were at first, because of that vicious assumption.”
—Ralph Waldo Emerson (1803–1882)
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“Natural selection, the blind, unconscious, automatic process which Darwin discovered, and which we now know is the explanation for the existence and apparently purposeful form of all life, has no purpose in mind. It has no mind and no mind’s eye. It does not plan for the future. It has no vision, no foresight, no sight at all. If it can be said to play the role of the watchmaker in nature, it is the blind watchmaker.”
—Richard Dawkins (b. 1941)