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In 2004 Florida was hit by four major hurricanes. In 2005 a survey indicated that, in \(2004,48 \%\) of the households in Florida had no plans for escaping an approaching hurricane. Suppose that a recent random sample of 50 households was selected in Gainesville and that those in 29 of the households indicated that their household had a hurricane escape plan. a. If the 2004 state percentages still apply to recent Gainesville households, use the Normal Approximation to Binomial Distribution applet to find the exact and approximate values of the probability that 29 or more of the households sampled have a hurricane escape plan. b. Refer to part (a). Is the normal approximation close to the exact binomial probability? Explain why.

Short Answer

Expert verified
Yes, the normal approximation is close if \(n\) and \(p(1-p)\) are large enough.

Step by step solution

01

Understand the Problem

First, we need to understand that we have historical data from 2004, where 48% of households in Florida had no hurricane escape plan. Thus, 52% had a plan. We are using a sample of 50 households chosen in Gainesville to see if at least 29 have a hurricane escape plan.
02

Establish the Binomial Parameters

Define the parameters: the probability of having a plan in 2004, \(p = 0.52\), and the sample size \(n = 50\). We are finding the probability that 29 or more out of 50 have a plan, which is a binomial situation: \(X \sim \text{Binomial}(n = 50, p = 0.52)\).
03

Calculate the Exact Binomial Probability

To find the exact probability of 29 or more households having a plan, we need to sum the individual probabilities from \(X = 29\) to \(X = 50\) using the binomial probability formula: \( P(X = k) = \binom{n}{k} p^k (1-p)^{n-k} \). This calculation can be done using statistical software or a binomial calculator.
04

Set Up the Normal Approximation

The binomial distribution \(X\) can be approximated by a normal distribution \(Y\) if \(n\) is large enough, using \( \mu = np \) and \( \sigma = \sqrt{np(1-p)} \). Here, \(\mu = 50 \times 0.52 = 26\) and \(\sigma = \sqrt{50 \times 0.52 \times 0.48} \approx 3.5\).
05

Use the Continuity Correction

To use the normal approximation, we apply a continuity correction. We calculate \( P(Y \geq 28.5) \) instead of \( P(Y \geq 29) \) since we are approximating a discrete distribution with a continuous one.
06

Calculate the Approximate Normal Probability

Convert to the standard normal distribution \(Z\) using \( Z = \frac{Y - \mu}{\sigma} \). For \(Y = 28.5\), \( Z = \frac{28.5 - 26}{3.5} \approx 0.71\). Use a standard normal table or calculator to find \( P(Z \geq 0.71) \).
07

Compare and Conclude

Compare the exact binomial probability from Step 3 with the approximate probability from Step 6 to check their closeness. The closer the results, the better the normal approximation is.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Binomial Distribution
The binomial distribution is a common probability distribution that captures outcomes from binary experiments. In our case, the experiment is determining if a household in Gainesville has a hurricane escape plan. The number of households surveyed is the sample size, which is 50 in this context. There are two possible outcomes: each household either does or does not have a plan.

The probability of success, defined as having a hurricane escape plan, is 52%, equivalent to 0.52. Therefore, we use the binomial random variable notation: \[X \sim \text{Binomial}(n = 50, p = 0.52)\]. With this setup, we can compute the probability of seeing 29 or more households with a plan by adding the probabilities of individual outcomes from 29 to 50 using the binomial probability formula:

\[P(X = k) = \binom{n}{k} p^k (1-p)^{n-k}\]

Applying this formula involves calculating combinations \(\binom{n}{k}\), raising probabilities to powers, and multiplying, which can easily be done with statistical software or binomial probability calculators.
Continuity Correction
A continuity correction is a technique used when a discrete distribution, like a binomial, is approximated by a continuous distribution, such as a normal distribution. Since the binomial distribution deals with discrete number of successes, it is essential to adjust for the fact that normal distribution is continuous.

In the problem, we are interested in the probability of 29 or more households having a hurricane escape plan. Without a continuity correction, applying normal approximation would have us calculate:\[ P(Y \geq 29) \]. However, using a continuity correction, we adjust this to \( P(Y \geq 28.5) \). This adjustment ensures that the approximation accounts for the discrete nature of the binomial distribution and provides a more accurate result when using the continuous normal curve.

By rounding down 0.5 from 29 to 28.5, we bridge the gap between discrete outcomes and continuous curve estimations, thereby refining the approximation process.
Standard Normal Distribution
The standard normal distribution is an essential concept in probability and statistics, often represented as a bell-shaped curve centered around a mean of zero and a standard deviation of one. It serves as a reference for converting normal random variables for easier probability computations.

In this exercise, after applying the continuity correction, we transform the normal approximation to standard form using the formula:
\[Z = \frac{Y - \mu}{\sigma}\]

Where \(Y\) is the continuity-corrected value (28.5), \(\mu\) the mean \((np = 26)\), and \(\sigma\) the standard deviation \((\sqrt{np(1-p)} \approx 3.5)\). For our problem, the transformation becomes:
\[Z = \frac{28.5 - 26}{3.5} \approx 0.71\]

This value of \(Z\) tells us how many standard deviations \(Y\) is above the mean. By using a standard normal table or calculator, we find \(P(Z \geq 0.71)\), which gives us the probability that at least 29 of the households have a hurricane escape plan.

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Most popular questions from this chapter

Suppose that \(Z\) has a standard normal distribution and that \(Y\) is an independent \(\chi^{2}\) -distributed random variable with \(\nu\) df. Then, according to Definition 7.2, $$T=\frac{Z}{\sqrt{Y / \nu}}$$ has a \(t\) distribution with \(\nu\) df. \(^{*}\) a. If \(Z\) has a standard normal distribution, give \(E(Z)\) and \(E\left(Z^{2}\right)\). [Hint: For any random variable, \(\left.E\left(Z^{2}\right)=V(Z)+(E(Z))^{2} \cdot\right]\) b. According to the result derived in Exercise \(4.112(\mathrm{a}),\) if \(Y\) has a \(\chi^{2}\) distribution with \(\nu\) df, then $$E\left(Y^{a}\right)=\frac{\Gamma([\nu / 2]+a)}{\Gamma(\nu / 2)} 2^{a}, \quad \text { if } \nu>-2 a$$. Use this result, the result from part (a), and the structure of \(T\) to show the following. [Hint: Recall the independence of \(Z \text { and } Y \)] i. \(E(T)=0,\) if \(\nu>1\) ii. \(V(T)=\nu /(\nu-2),\) if \(\nu>2\)

Access the applet Normal Approximation to Binomial Distribution (at https://college.cengage.com/nextbook/statistics/wackerly 966371/student/html/index.html). When the applet is started, it displays the details in Example 7.11 and Figure 7.9 . Initially, the display contains only the binomial histogram and the exact value (calculated using the binomial probability function) for \(p(8)=P(Y=8) .\) Scroll down a little and click the button "Toggle Normal Approximation" to overlay the normal density with mean 10 and standard deviation \(\sqrt{6}=2.449\) the same mean and standard deviation as the binomial random variable \(Y\). You will get a graph superior to the one in Figure \(7.9 .\) a. How many probability mass or density functions are displayed? b. Enter 0 in the box labeled "Begin" and press the enter key. What probabilities do you obtain? c. Refer to part (b). On the line where the approximating normal probability is displayed, you see the expression Normal: $$ P(-0.5<=k<=8.5)=0.2701 $$ Why are the .5s in this expression?

A forester studying the effects of fertilization on certain pine forests in the Southeast is interested in estimating the average basal area of pine trees. In studying basal areas of similar trees for many years, he has discovered that these measurements (in square inches) are normally distributed with standard deviation approximately 4 square inches. If the forester samples \(n=9\) trees, find the probability that the sample mean will be within 2 square inches of the population mean.

The fracture strength of tempered glass averages 14 (measured in thousands of pounds per square inch) and has standard deviation 2. a. What is the probability that the average fracture strength of 100 randomly selected pieces of this glass exceeds \(14.5 ?\) b. Find an interval that includes, with probability \(0.95,\) the average fracture strength of 100 randomly selected pieces of this glass.

Refer to Exercise \(7.34 .\) Suppose that \(F\) has an \(F\) distribution with \(\nu_{1}=50\) numerator degrees of freedom and \(\nu_{2}=70\) denominator degrees of freedom. Notice that Table 7 , Appendix 3 , does not contain entries for 50 numerator degrees of freedom and 70 denominator degrees of freedom. a. What is \(E(F) ?\) b. Give \(V(F)\) c. Is it likely that \(F\) will exceed \(3 ?[\text { Hint: Use Tchebysheff's theorem. }]\)

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