Math Problem Statement

The mean height of women in the United States is 62.35 inches, with a standard deviation is 5.69. A random sample of 50 women is selected. What is the probability that the mean height for the sample is a. Less than 61 inches? b. Between 61.5 inches and 63 inches? c. Taller than 62 inches

Solution

To solve this problem, we use the concept of the sampling distribution of the sample mean. The sampling distribution of the sample mean follows a normal distribution with the following parameters:

  1. Mean (μ): Equal to the population mean, μ=62.35\mu = 62.35 inches.
  2. Standard deviation (σ): Equal to the population standard deviation divided by the square root of the sample size, σxˉ=σn=5.6950\sigma_{\bar{x}} = \frac{\sigma}{\sqrt{n}} = \frac{5.69}{\sqrt{50}}.

Let's calculate σxˉ\sigma_{\bar{x}}:

σxˉ=5.69500.8046\sigma_{\bar{x}} = \frac{5.69}{\sqrt{50}} \approx 0.8046

Thus, the sampling distribution of the sample mean is:

XˉN(62.35,0.8046)\bar{X} \sim N(62.35, 0.8046)

We will calculate the required probabilities using the standard normal distribution ZZ, where:

Z=xˉμσxˉZ = \frac{\bar{x} - \mu}{\sigma_{\bar{x}}}


(a) Probability that the sample mean is less than 61 inches

Find P(Xˉ<61)P(\bar{X} < 61):

  1. Compute the ZZ-score: Z=6162.350.8046=1.350.80461.678Z = \frac{61 - 62.35}{0.8046} = \frac{-1.35}{0.8046} \approx -1.678

  2. Use the standard normal table or a calculator to find P(Z<1.678)P(Z < -1.678): P(Z<1.678)0.0468P(Z < -1.678) \approx 0.0468

Thus, the probability is:

P(Xˉ<61)0.0468(4.68%)P(\bar{X} < 61) \approx 0.0468 \, (4.68\%)


(b) Probability that the sample mean is between 61.5 inches and 63 inches

Find P(61.5<Xˉ<63)P(61.5 < \bar{X} < 63):

  1. Compute the ZZ-scores: Z1=61.562.350.8046=0.850.80461.056Z_1 = \frac{61.5 - 62.35}{0.8046} = \frac{-0.85}{0.8046} \approx -1.056 Z2=6362.350.8046=0.650.80460.808Z_2 = \frac{63 - 62.35}{0.8046} = \frac{0.65}{0.8046} \approx 0.808

  2. Use the standard normal table or a calculator to find the probabilities: P(Z<1.056)0.1455,P(Z<0.808)0.7891P(Z < -1.056) \approx 0.1455, \quad P(Z < 0.808) \approx 0.7891

  3. Subtract the probabilities: P(61.5<Xˉ<63)=P(Z<0.808)P(Z<1.056)P(61.5 < \bar{X} < 63) = P(Z < 0.808) - P(Z < -1.056) P(61.5<Xˉ<63)0.78910.1455=0.6436(64.36%)P(61.5 < \bar{X} < 63) \approx 0.7891 - 0.1455 = 0.6436 \, (64.36\%)


(c) Probability that the sample mean is taller than 62 inches

Find P(Xˉ>62)P(\bar{X} > 62):

  1. Compute the ZZ-score: Z=6262.350.8046=0.350.80460.435Z = \frac{62 - 62.35}{0.8046} = \frac{-0.35}{0.8046} \approx -0.435

  2. Use the standard normal table or a calculator to find P(Z>0.435)P(Z > -0.435): P(Z>0.435)=1P(Z<0.435)P(Z > -0.435) = 1 - P(Z < -0.435) P(Z<0.435)0.3327(from the table)P(Z < -0.435) \approx 0.3327 \quad \text{(from the table)} P(Z>0.435)10.3327=0.6673(66.73%)P(Z > -0.435) \approx 1 - 0.3327 = 0.6673 \, (66.73\%)


Final Answers:

a. P(Xˉ<61)0.0468(4.68%)P(\bar{X} < 61) \approx 0.0468 \, (4.68\%)
b. P(61.5<Xˉ<63)0.6436(64.36%)P(61.5 < \bar{X} < 63) \approx 0.6436 \, (64.36\%)
c. P(Xˉ>62)0.6673(66.73%)P(\bar{X} > 62) \approx 0.6673 \, (66.73\%)


Would you like further details on these calculations?


Relative Questions:

  1. How does the Central Limit Theorem justify the use of a normal distribution here?
  2. What would happen to σxˉ\sigma_{\bar{x}} if the sample size were increased?
  3. Why do we use the standard normal distribution for probabilities in this problem?
  4. How can we interpret a Z-score in this context?
  5. What if the population distribution were not normal—would the results change?

Tip: Always check whether the sample size is large enough for the Central Limit Theorem to apply, especially if the population distribution is not normal!

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Math Problem Analysis

Mathematical Concepts

Statistics
Probability
Sampling Distribution
Normal Distribution
Central Limit Theorem

Formulas

Z = (X̄ - μ) / (σ / √n)
Standard error = σ / √n

Theorems

Central Limit Theorem
Properties of the Normal Distribution

Suitable Grade Level

Grades 11-12 or College-level Statistics