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ASK THE PROFESSOR FORUM

Course Documents

Chapter 1 - Intro

Chapter 2 - Methods for Describing Sets of Data

Chapter 3 - Probability

Chapter 4 - Discrete Random Variables

Chapter 5 - Normal Random Variables

Chapter 6 - Sampling Distributions

Chapter 7 - Confidence Intervals

Chapter 8 - Tests of Hypothesis: One Sample

Chapter 9 - Confidence Intervals and Hypothesis Tests: Two Samples

Sample Exam I: Chapters 1 & 2

Sample Exam II: Chapters 3 & 4

Sample Exam III: Chapters 5 & 6

Sample Exam IV: Chapters 7 & 8

Ask the Professor Forum

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 ?

Posted to STATS 2 on Monday, September 22, 2014   Replies: 1


Professor Mcguckian
09/22/2014
10:53 PM EST

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

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