The Professor's Response

Hi Professor!

I have a simple question, that I got stuck on, I have watched stats professor (the video) and I understand what I needed to do for Sect. 8.4 #30 but I don't understand why? to get the P-value I was under the impression you would subtract 0.50 minus the shaded area on the Z-table? but in that problem we added the amount to 0.50. why did we do that ?

See the professor's answer below.

Hi Rasheeda,

You might want to watch the video more closely. I always explain why I perform the steps I do when working out a problem. In the video, I reiterate the rule for finding p-values. This begins about 39 seconds into the video. The answer you are looking for is there.

There is no rule that says you need to subtract the table value from 0.5000. That is how the p-value is found in many problems, but that is only because the test statistic is usually on the side of the curve that corresponds to the type of test we are conducting. That technique is not part of the definition of the p-value. You must use the rule described in the video and in class. To learn to use this rule, you must pay close attention to the type of test you are dealing with, and that can be found by looking at the alternative hypothesis. Remember, if the alternative hypothesis uses a less than (<) symbol, you must find the area to the left of the test statistic. If the alternative hypothesis has a greater than (>) symbol, you must find the area to the right of the test statistic, and when the alternative hypothesis has a not equal to symbol, you find twice the tail area.

I answered this same question for another student in these forums, so you might also view my earlier response here:

http://www.statsprofessor.com/atpquestion.php?qid=137

Let me know if you still need help after rewatching the relevant part of the video.

Professor McGuckian