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25. Did you vote? A sample survey contacted an SRS of 663 registered voters in Oregon shortly after an election and asked respondents whether they had voted. Voter records show that 56%of registered voters had actually voted. We will see later that in repeated random samples of size 663, the proportion in the sample who voted (call this proportion V) will vary according to the Normal distribution with mean μ=0.56 and standard deviation σ=0.019.
(a) If the respondents answer truthfully, what is P(0.52≤V≤0.60)? This is the probability that the sample proportion V estimates the population
proportion 0.56 within ±0.04.
(b) In fact, 72%of the respondents said they had voted (V=0.72). If respondents answer truthfully, what is P(V≥0.72)? This probability is so small that it is good evidence that some people who did not vote claimed that they did vote.

Short Answer

Expert verified

(a) The probability of P(0.52≤V≤0.60) is 0.9652.

(b) The probability is P(V≥0.72), and it could be claimed.

Step by step solution

01

Part (a) Step 1: Given information 

The probability that the sample proportionVestimates the population proportion 0.56within ±0.04.

02

Part (a) Step 2: Explanation 

The probability that the sample proportion Vwill accurately predict the population within±0.04is determined as follows:

P(0.52≤V≤0.60)=P0.52-μσ≤x-μσ≤0.60-μσ

Here, population mean is (μ)=0.56, and the population standard is deviation(σ)=0.019

=P0.52-0.560.019≤x-μσ≤0.60-μσ=P(−2.11≤Z≤2.11)=0.9652

03

Part (b) Step 1: Given information

Let, 72%of the respondents said they had voted (V=0.72).

To find the probability for P(V≥0.72)to check, could be claimed by the people who did not vote.

04

Part (b) Step 2: Explanation

By using the normal probability table to calculate the probability as:

P(V≥0.72)=P(x-μσ≥0.72-μσ)=PZ≥0.72−0.560.019=P(z≥8.42)=0.0001

As a result, the chance is too low. It's possible to make a case for it. Because getting a sample proportion of 72percent by random is virtually hard if the genuine percentage is 56percent .

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Most popular questions from this chapter

85. Aircraft engines Engineers define reliability as the probability that an item will perform its function under specific conditions for a specific period of time. A certain model of aircraft engine is designed so that each engine has probability 0.999 of performing properly for an hour of flight. Company engineers test an SRS of 350 engines of this model. Let X= the number that operate for an hour without failure.
(a) Explain whyX is a binomial random variable.

(b) Find the mean and standard deviation ofX. Interpret each value in context.
(c) Two engines failed the test. Are you convinced that this model of engine is less reliable than it’s supposed to be? Compute P(X≤348) and use the result to justify your answer.

Ms. Hall gave her class a 10-question multiple-choice quiz. Let X=the number of questions that a randomly selected student in the class answered correctly. The computer output below gives information about the probability distribution of X. To determine each student’s grade on the quiz (out of 100), Ms. Hall will multiply his or her number of correct answers by 10. Let G=the grade of a randomly chosen student in the class.

NMeanMedianStDevMinMaxQ1Q3307.68.51.3241089

(a) Find the median of G. Show your method.

(b) Find the IQR of G. Show your method.

(c) What shape would the probability distribution of Ghave? Justify your answer

Spell-checking Refer to Exercise 3. Calculate the mean of the random variable X and interpret this result in context.

The deck of 52cards contains 13hearts. Here is another wager: Draw one card at random from the deck. If the card drawn is a heart, you win 2. Otherwise, you lose 1. Compare this wager (call it Wager 2) with that of the previous exercise (call it Wager 1). Which one should you prefer?

(a) Wager 1, because it has a higher expected value.

(b) Wager 2, because it has a higher expected value.

(c) Wager 1, because it has a higher probability of winning.

(d) Wager 2, because it has a higher probability of winning. (e) Both wagers are equally favorable

18. Life insurance

(a) It would be quite risky for you to insure the life of a 21-year-old friend under the terms of Exercise 14. There is a high probability that your friend would live and you would gain \(1250in premiums. But if he were to die, you would lose almost \)100,000. Explain carefully why selling insurance is not risky for an insurance company that insures many thousands of 21-year-old men.

(b) The risk of an investment is often measured by the standard deviation of the return on the investment. The more variable the return is, the riskier the
investment. We can measure the great risk of insuring a single person’s life in Exercise 14by computing the standard deviation of the income Y that the insurer will receive. Find σY using the distribution and mean found in Exercise 14.

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