When we have to randomly but fairly choose between two outcomes, we instinctively reach for the coin toss. It is because it is an article of faith that the two outcomes of heads and tails are equally likely. But the two sides of the coin are not identical, and hence that slight difference may make a difference in outcome probabilities. In fact, there are four possible forms of bias that may exist. It is possible that either heads or tails may come out on top slightly more frequently or that there is a same-side bias (i.e., the side that is on top when flipped is more likely to be on top when it falls) or an opposite side bias.
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