Suppose you wish to investigate whether your spun tennis racquet is equally likely to land with the label facing up or down. Does this study call for a one-sided or two-sided alternative?
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One-sided, because there is only one variable: how the label lands.
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Two-sided, because there are two possible outcomes: up or down.
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One-sided, because you want to know whether the label is more likely to land face up.
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Two-sided, because you want to know whether the spinning process is fair or biased in either direction.