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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

Hi professor,

I got number #24 wrong in section 4.4. these are my numbers but I think they are mixed up ?
n=50
x=4
p=.35
q=.65

Posted to STATS 1 on Friday, October 18, 2013   Replies: 1


Professor Mcguckian
10/18/2013
10:15 PM EST

Hi Tufic,

In Binomial probability, n = the number of trials, so in this case it represents the number of problems we will take a guess on. You are correct to label n = 50. 

We are looking for the probability that we get X successes. X represents the number of successes we want to have (note: a success isn't necessarily something positive). In this case, the problem says we want to know the likelihood of missing only 35 questions. This means for us missing a question is a success, and we want to know the probability that we miss 35 questions.  This means X = 35 because 35 missed questions would be 35 successes.  

Now this next part is important: if we define a success as missing a question, p = the probability we have a success in a single trial.  This means to find p, we need to determine the probability that we miss a single question when we take a guess on one.  Well, the problem says we have four answer choices for each question.  This means the chance we guess incorrectly is P(missed question) = # of wrong answer options/ total number of answer options. This should be P(missed question) = 3 / 4, because there are 3 wrong answer choices for every question and four total answer choices for each one.

Lastly, our q = 1 - p, so in this case, that is q = 1 - 3/4 = 1/4. 

This answer to the problem becomes: 50 C 35 * (3/4)^35 * (1/4)^15   (note: we have 15 as the exponent for the 1/4 because if we miss 35 questions, we get 15 correct out of 50).

Hope that helps,

Professor McGuckian

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