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The key to understanding the difference between one and two tailed hypotheses lies in understanding the concept of the direction of a relationship. Direction refers to the manner in which we are expecting our differences to occur.
Consider the picture above. This shows us that there are two ways in which our groups can be different. 1) Belize children can be further from their mothers than children in Samoa or 2) Samoan children can be further from their mothers than children in Belize. These two ways to be different comprise the two "tails" of our distribution. To decide whether we have a one or two tailed hypothesis, all we have to do is decide how many of our tails (or ways to be different) are considered in our hypothesis statement.
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