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In \(2008,\) the murder rates (per 100,000 residents) for the 50 states and the District of Columbia (D.C.) had a mean of 5.39 and a standard deviation of 4.434 (Statistical Abstract of the United States). a. D.C. had a murder rate of 31.4 . Find its \(z\) -score. If the distribution were roughly normal, would this be unusually high? Explain. b. Based on the mean and standard deviation, do you think that the distribution of murder rates is approximately normal? Why or why not?

Short Answer

Expert verified
a. The z-score is approximately 5.87, which is unusually high. b. The distribution is likely not normal due to extreme outliers like D.C.'s rate.

Step by step solution

01

Understanding the Z-Score Formula

The formula for calculating the z-score of a data point is: \( z = \frac{(X - \mu)}{\sigma} \), where \( X \) is the data point, \( \mu \) is the mean of the data set, and \( \sigma \) is the standard deviation.
02

Applying the Formula for D.C.

For D.C., the murder rate \( X \) is 31.4. The mean \( \mu \) is 5.39, and the standard deviation \( \sigma \) is 4.434. Plug these values into the formula: \[ z = \frac{(31.4 - 5.39)}{4.434} \]
03

Calculating the Z-Score

Perform the arithmetic operations: \( 31.4 - 5.39 = 26.01 \). Then divide by the standard deviation: \( z = \frac{26.01}{4.434} \approx 5.87 \). Thus, the z-score for D.C. is approximately 5.87.
04

Interpreting the Z-Score

A z-score of 5.87 is very high, indicating that the murder rate for D.C. is 5.87 standard deviations above the mean. If the distribution were roughly normal, a z-score above 2 or 3 is usually considered unusual, making this rate extremely unusual.
05

Assessing Normality of Distribution

The very high z-score suggests that the distribution is likely not normal because in a normal distribution, data points are generally within 2 to 3 standard deviations of the mean. Thus, the presence of such an extreme outlier (D.C.'s murder rate) indicates the distribution could be skewed.

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Key Concepts

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

Murder Rate Distribution
Murder rate distribution is a way to describe how different regions or areas compare in terms of the number of murders per a specified population size, often reported per 100,000 residents. In the context of the United States for the year 2008, this measurement helps us understand how states compare against each other regarding safety and crime risk.

For this particular data set, Washington D.C. had a much higher murder rate than most other states. This suggests that D.C. was an outlier in the distribution, differing significantly from the mean. Understanding murder rate distribution helps policy makers and law enforcement officials allocate resources and create strategies to address crime and safety issues more effectively. By analyzing the distribution, they can identify areas most in need of intervention and potentially explore the underlying causes behind such disparities.
Normal Distribution
A normal distribution, often represented as a bell-shaped curve, is a statistical concept where data points are symmetrically distributed around the mean. Here's what you need to know about normal distribution:

  • Most data points fall near the mean.
  • The mean, median, and mode are all the same point.
  • About 68% of the data lies within one standard deviation from the mean, 95% within two, and 99.7% within three standard deviations.


This distribution property makes it easier to predict outcomes and analyze the data. However, when there's a significant outlier, like the D.C. murder rate, it's a sign that the distribution may not be normal. The presence of extremely high or low values suggests the data might be skewed, hence not fitting a normal distribution pattern. Understanding whether your data follows a normal distribution is essential for applying many statistical analysis techniques accurately.
Standard Deviation
Standard deviation is a crucial statistical measure that tells us how much the individual data points in a set differ from the mean.

  • A small standard deviation means data points are close to the mean, implying low variability.
  • A large standard deviation indicates a wide spread of data points, showing high variability within the data set.
For the 2008 murder rates, a standard deviation of 4.434 reflects the typical difference between each state's murder rate and the overall mean of 5.39 per 100,000 residents.

This tool helps us understand the variability in murder rates across different states. When the murder rate of Washington D.C. stands at 31.4 with such a deviation, it significantly deviates from most other data points. The greater the standard deviation compared to the mean, the more spread out the data and less predictable it is. In public safety, understanding standard deviation aids in recognizing abnormal patterns which could necessitate closer investigation or action.
Mean of Data Set
The mean of a data set represents a central or typical value, and it's calculated by summing up all the data points and dividing by their number. For the murder rates in the 50 states and D.C. in 2008, the mean value was 5.39 murders per 100,000 people.

Understanding the mean allows us to view the overall level of murder rates across the country. It provides a baseline to identify which states are above or below average. Calculation and interpretation of the mean can help to spot trends or unusual values.

For D.C., with a murder rate of 31.4, the mean helps us understand just how far removed this rate is from the typical value. This large deviation hints at underlying factors affecting D.C. that might not be present elsewhere, prompting a deeper exploration into the factors contributing to such high rates.

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Most popular questions from this chapter

The Internet site www.ItsJustLunch .com advertises itself as a dating service for busy professionals that has set up over two million first dates for lunch or drinks after work. An advertisement for this site stated that a survey of their users found that a woman has chance 1 in 8 of a second date if she has not heard from the man within 24 hours of their first date. On Saturday, Shawna had a luncheon date with Jack and a dinner date with Lawrence. By Sunday evening she had not heard from either of them. Based on the information claimed by www.ItsJustLunch.com, construct a table with the probability distribution of \(X=\) the number of these men 2) with whom she has a second date. (Source: \((0,1,\) or Background information from www.ItsJustLunch.com.)

Consider a game of poker being played with a standard 52 -card deck (four suits, each of which has 13 different denominations of cards). At a certain point in the game, six cards have been exposed. Of the six, four are diamonds. Your opponent makes a bet of \(\$ 20\), and you must decide whether to call the bet. If you do call the bet, you will receive one more card. If that final card turns out to be another diamond, you will win \(\$ 100\). If not, you will lose the hand as well as the \(\$ 20\) you called in order to receive the final card. On the other hand, if you do not call the bet, the hand ends immediately, your opponent wins, and you neither win nor lose any more money. a. Specific the probability distribution for \(X=\) expected winnings. b. Find the expected value of \(X\). Based on the expected value, should you call the \(\$ 20\) bet and receive one more card or not call the bet?

A random number generator is used to generate a real number between 0 and 1 , equally likely to fall anywhere in this interval of values. (For instance, \(0.3794259832 \ldots\) is a possible outcome.) a. Sketch a curve of the probability distribution of this random variable, which is the continuous version of the uniform distribution (see Exercise 6.1). b. What is the mean of this probability distribution? c. Find the probability that this random variable falls between 0.25 and 0.75 .

A social scientist uses the General Social Survey to study how much time per day people spend watching TV. The variable denoted by TVHOURS at the GSS Web site measures this using the values \(0,1,2, \ldots, 24\) a. Explain how, in theory, TV watching is a continuous random variable. b. An article about the study shows two histograms, both skewed to the right, to summarize TV watching for females and males. Since TV watching is in theory continuous, why were histograms used instead of curves? c. If the article instead showed two curves, explain what they would represent.

Selling houses Let \(X\) represent the number of homes a real estate agent sells during a given month. Based on previous sales records, she estimates that \(\mathrm{P}(0)=0.68\),\(\mathrm{P}(1)=0.19, \mathrm{P}(2)=0.09, \mathrm{P}(3)=0.03, \mathrm{P}(4)=0.01\) with negligible probability for higher values of \(x\). a. Explain why it does not make sense to compute the mean of this probability distribution as \((0+1+2+3+4) / 5=2.0\) b. Find the correct mean.

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