The correct answer is: A. Power of a one sided test is lower than the power of the associated two sided test.
The power of a test is the probability of rejecting the null hypothesis when the alternative hypothesis is true. In other words, it is the probability of correctly identifying a difference when there actually is a difference.
A one-sided test is a test in which the alternative hypothesis is stated in terms of a difference in one direction. For example, a one-sided test might be used to test the hypothesis that the mean of a population is greater than a certain value.
A two-sided test is a test in which the alternative hypothesis is stated in terms of a difference in either direction. For example, a two-sided test might be used to test the hypothesis that the mean of a population is different from a certain value.
The power of a one-sided test is always lower than the power of the associated two-sided test. This is because a one-sided test is more conservative than a two-sided test. A one-sided test only rejects the null hypothesis when the evidence is very strong, while a two-sided test rejects the null hypothesis when the evidence is moderately strong.
For example, suppose we are conducting a one-sided test to test the hypothesis that the mean of a population is greater than 100. The null hypothesis is that the mean is equal to 100, and the alternative hypothesis is that the mean is greater than 100. The power of this test is the probability of rejecting the null hypothesis when the true mean is actually 101.
Now suppose we are conducting a two-sided test to test the same hypothesis. The null hypothesis is still that the mean is equal to 100, but the alternative hypothesis is that the mean is either greater than 100 or less than 100. The power of this test is the probability of rejecting the null hypothesis when the true mean is actually 101.
As you can see, the power of the one-sided test is lower than the power of the two-sided test. This is because the one-sided test is more conservative. It only rejects the null hypothesis when the evidence is very strong, while the two-sided test rejects the null hypothesis when the evidence is moderately strong.
In conclusion, the correct answer is: A. Power of a one sided test is lower than the power of the associated two sided test.