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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

80% confidence level = 1.282 Stat 2

that is the answer that the book gave. Where did the extra 2 come from because the answer i got is 1.28

Posted to STATS 2 on Tuesday, June 23, 2015   Replies: 1

Professor Mcguckian
10:00 AM EST

Hi Emily,

You probably got 1.28 from the z table, which makes sense since you are looking for a z value; however, in order to ensure that you get the most precise answer available, you should use the t table to find this value.

Whenever the t table can be used, you should use it, since it provides an extra decimal place. It is also easier to use than the z table. The confidence levels that can be found on the t table (in order, going left to right) are: 99%, 98%, 95%, 90%, and 80%. If you go to the t table with alpha in one tail, you will find the value 1.282. Remember, that at the top of the t table, you can look up the alpha in one tail. In this case, that is 20%/2 = 10% = 0.10. If we go to that column of the table and then go to the last entry in that column, we find our critical z value, 1.282. 

See the video below to see how this is done in general:


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