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Showers According to home-water-works.org, the average shower in the United States lasts \(8.2\) minutes. Assume the standard deviation of shower times is 2 minutes and the distribution of shower times is right-skewed. Which of the following questions can be answered using the Central Limit Theorem for sample means as needed? If the question can be answered, do so. If the question cannot be answered, explain why the Central Limit Theorem cannot be applied. a. Find the probability that a randomly selected shower lasts more than 9 minutes. b. If five showers are randomly selected, find the probability that the mean length of the sample is more than 9 minutes. c. If 50 showers are randomly selected, find the probability that the mean length of the sample is more than 9 minutes.

Short Answer

Expert verified
a. The Central Limit Theorem cannot be applied. b. The Central Limit Theorem cannot be applied because the sample size is too small. c. The probability that 50 showers will have a mean length greater than 9 minutes is approximately 0.0021 or 0.21%.

Step by step solution

01

Problem a: Shower lasting more than 9 minutes

This question is not about the average of a sample, but a single observation. Therefore, the Central Limit Theorem doesn't apply here because it concerns the behavior of the sample mean. Thus, we cannot answer this question with the information given.
02

Problem b: Average of 5 Showers

Although the Central Limit Theorem can start to be useful with small sample sizes, 5 is too small for the theorem to apply, especially given that our population distribution is not normal (it's right-skewed), so we cannot answer this question using the Central Limit Theorem.
03

Problem c: Average of 50 Showers

This question can be answered using the Central Limit Theorem since 50 is a large sample size. According to the theorem, regardless of the characteristics of the original distribution, the distribution of the sample mean is approximately normal with mean equal to the population mean, and standard deviation equal to the population standard deviation divided by the square root of the sample size. Here, we need to find the chance that the mean shower time of 50 showers exceeds 9 minutes. To do that, we first calculate the standard deviation of the sample which is \( \frac{2}{\sqrt{50}} \approx 0.28 \). The z-score for the sample mean of 9 minutes is: \( z = \frac{9 - 8.2}{0.28} \approx 2.857 \). To find the probability that our sample mean is greater than 9, we need to find the right tail from the z-score above. Using the standard normal distribution tables, this can be approximately calculated as \(1-P(Z<2.857)\). Using the z-table gives us that \(P(Z<2.857) \approx 0.9979\). So, \( 1- 0.9979 = 0.0021 \). Therefore, the probability that 50 showers will have a mean length greater than 9 minutes is approximately 0.0021 or 0.21%.

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Key Concepts

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

Probability Distribution
A probability distribution is a mathematical function that provides the probabilities of occurrence of different possible outcomes in an experiment. For a particular situation, this distribution can take various shapes, such as symmetric, bell-shaped normal distributions or skewed distributions.

When talking about a right-skewed distribution, as mentioned with the shower times, it means that the tail of the distribution extends more to the right - in other words, there are a fewer number of showers that last much longer than average. This characteristic affects the application of statistical methods, such as the Central Limit Theorem, which predicts the behavior of the sampling distribution based on the original population distribution.
Sample Mean
The sample mean, also known as the arithmetic average, is calculated by adding all the observations from a sample and dividing by the number of observations. It represents the central or 'typical' value of the sample data. When looking at multiple samples, the distribution of sample means can also be analyzed to infer the population mean.

For instance, in our exercise, if we were to repeatedly take samples of 50 showers, we would calculate the mean for each sample and all these means would form their own distribution. According to the Central Limit Theorem, this distribution of sample means tends to normalize as the sample size increases, even if the original population distribution is skewed.
Skewed Distribution
A skewed distribution refers to a probability distribution that is asymmetric and deviates from the symmetric bell-shaped normal distribution. It can be either right (positive) skewed or left (negative) skewed, depending on the direction of the longer tail. In the context of the problem, the distribution of shower times is right-skewed, which means fewer instances of very long showers and a concentration of shower times much shorter than the mean.
Standard Deviation
The standard deviation is a measure of the amount of variation or dispersion in a set of values. It quantifies the average distance that the dataset diverges from the mean and characterizes the width of the probability distribution. A low standard deviation means that the values tend to be close to the mean, while a high standard deviation indicates that the data points can be spread out over a wide range of values.

In this scenario, the standard deviation of the shower times in the general population is given as 2 minutes. When we calculate the probability involving sample means (as in part c of the exercise), we must adjust the standard deviation by dividing it by the square root of the sample size to account for the reduced variability expected with larger sample sizes.
Z-score
A z-score indicates how many standard deviations an element is from the mean. In other words, it's a numerical measurement that describes a value's relationship to the mean of a group of values. The z-score is a crucial part of finding probabilities in a normal distribution and for applying the Central Limit Theorem.

For the final part of our exercise, the z-score was used to determine how unusual or typical a sample mean of 9 minutes was, based on the normalized distribution of sample means. By calculating the z-score and referencing the standard normal distribution tables, we found the probability that the sample mean of 50 showers exceeds 9 minutes.

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Most popular questions from this chapter

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