A one-tailed test is directional. For example, let us assume your hypothesis is the mean weight of all Baseball payers is 5' 9''. The alternative hypothesis could be (1) it is less than 5' 9'' or (2) grater than 5'9''. It specifies a direction. The test is designed so that the criteria uses either the upper or lower part of a distribution. A two-taled test does not specify a direction. You want to know whether the average height is either 5'9'' or not (doesn't matter which way). The test is designed so that the criteria uses both the upper and lower part of a distribution
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The choice of one-tailed or two-tailed tests follows the logic of the hypothesis that is being tested! The one-tailed test, if appropriate, will be more powerful.
You could use a two-tailed t-test. You would use a two-tailed test instead of a one-tailed test because you are not hypothesizing which direction the difference will be. If you hypothesize before hand the direction of change, you could use a one-tailed test.
· One-tailed test looks at the probability that the sample mean was either "greater than", or "less than or equal to" · Two-tailed test, sees if two means are different from each other (ie from different populations), or from the same population and tries to establish "equal to" or "not equal to
The difference between one hundred thousand and one million is one in ten, or 0.1.