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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

Hello, Mr. McGuckian. For the homework problems in 9.8 (#35 and 36), I am getting he correct answer but the wrong sign. For example for #36, the answer is (1.21, 7.54). But I got (-1.21, -7.54). I was wondering why this was.
Thanks.

Posted to STATS 2 on Wednesday, October 8, 2014   Replies: 4


Professor Mcguckian
10/08/2014
9:01 AM EST

Hi Carmen,

This is a result of performing your subtraction in the opposite order of how I performed my subtraction. This starts with your expression of the claim and how you relate the two means in that step. Watch the video below for a general explanation of what is causing the different signs, and why it doesn't matter.

Professor McGuckian


Professor Mcguckian
10/08/2014
9:12 AM EST

Hi Carmen,

For the specific question you are asking about, the subtraction of the before and after matters greatly. If your subtraction was done First Score - Second Score but mine was done Second Score - First Score, your answer will be opposite of mine. The interpretation is what matters though, not the order. If your interval is negative and you did First - Second, the interpretation is that the Second set of scores is larger (since the interval is negative). In my interval it is entirely positive, so I would say the first mean is larger, but the first mean in my subtraction is the Second score. In the end, we both have the exact same result. It just looks different in terms of the sign because of the different order of the subtraction.

Hope that helps,

Professor McGuckian


.
10/10/2014
8:47 PM EST
Hi Professor,

If you have a negative and a positive interval that means that there is no difference correct?

Professor Mcguckian
10/11/2014
1:17 PM EST

That is correct Sandra. There is not a statistically significant difference between the two population means. 

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