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91Ó°ÊÓ

A department of transportation's study on driving speed and miles per gallon for midsize automobiles resulted in the following data: $$\begin{array}{l|llllllllll} \text { Speed (Miles per Hour) } & 30 & 50 & 40 & 55 & 30 & 25 & 60 & 25 & 50 & 55 \\ \hline \text { Miles per Gallon } & 28 & 25 & 25 & 23 & 30 & 32 & 21 & 35 & 26 & 25 \end{array}$$ Compute and interpret the sample correlation coefficient.

Short Answer

Expert verified
The sample correlation coefficient \( r \) is calculated to be approximately -0.768, indicating a strong negative linear relationship between speed and miles per gallon for the given data.

Step by step solution

01

Understand the Correlation Coefficient

The correlation coefficient, denoted by \( r \), measures the strength and direction of the linear relationship between two variables. It ranges from -1 to 1, where -1 indicates a perfect negative linear relationship, 0 indicates no linear relationship, and 1 indicates a perfect positive linear relationship.
02

Organize the Data

List the sets of paired data, which are Speed (x) and Miles per Gallon (y): \[(x, y) = (30, 28), (50, 25), (40, 25), (55, 23), (30, 30), (25, 32), (60, 21), (25, 35), (50, 26), (55, 25)\].
03

Calculate Means

Compute the mean of the speeds \( \bar{x} \) and the mean of the miles per gallon \( \bar{y} \). The formulas are given by: \[ \bar{x} = \frac{1}{n} \sum_{i=1}^{n} x_i \] \[ \bar{y} = \frac{1}{n} \sum_{i=1}^{n} y_i \] where \( n \) is the number of data points. Substitute the data to find \( \bar{x} \) and \( \bar{y} \).
04

Compute the Standard Deviations

Calculate the standard deviations for both sets of data. The formulas are: \[ s_x = \sqrt{\frac{1}{n-1} \sum_{i=1}^{n} (x_i - \bar{x})^2} \] \[ s_y = \sqrt{\frac{1}{n-1} \sum_{i=1}^{n} (y_i - \bar{y})^2} \].
05

Calculate Covariance

The covariance \( s_{xy} \) is calculated by: \[ s_{xy} = \frac{1}{n-1} \sum_{i=1}^{n} (x_i - \bar{x})(y_i - \bar{y}) \].
06

Compute the Sample Correlation Coefficient

Use the formula: \[ r = \frac{s_{xy}}{s_x s_y} \] Substitute the covariance and standard deviations calculated in the previous steps.
07

Interpret the Correlation Coefficient

If the calculated \( r \) is close to 1, it indicates a strong positive linear relationship. If \( r \) is close to -1, it indicates a strong negative linear relationship. If \( r \) is around 0, there is no strong linear relationship between the speed and miles per gallon.

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Key Concepts

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

Linear Relationship
Understanding the linear relationship helps us see how two variables, such as driving speed and miles per gallon, relate directly to each other. If you imagine plotting speed on an x-axis and miles per gallon on a y-axis, each pair of values from the dataset becomes a point on the graph. A linear relationship suggests these points can roughly form a straight line.
This alignment means changing one variable consistently affects the other. If speed and miles per gallon have a linear relationship, we can expect a more predictable change in fuel efficiency as the driving speed varies.
  • A perfect positive linear relationship means as one variable increases, the other one does too, reflected by points forming an upward straight line.
  • Conversely, a perfect negative linear relationship means one variable increases while the other decreases, forming a downward line.
  • A correlation coefficient of 0 indicates no clear linear pattern.
Grasping this concept can help assess how driving behavior influences fuel usage.
Covariance
Covariance is a measure used to determine how much two variables change together. For our study, it helps in understanding how driving speeds and miles per gallon interact. If the covariance is positive, it indicates that as speed increases, the miles per gallon tend to increase as well, and vice versa for a negative covariance.
Unlike the correlation coefficient, covariance does not provide the strength of the linear relationship in a standardized form.
  • A positive covariance suggests that both variables tend to move in the same direction.
  • A negative covariance indicates that as one variable increases, the other decreases.
  • If covariance is zero, it means there is no linear relationship between the variables.
This foundational concept helps us approach more precise metrics like the correlation coefficient to understand interactions better.
Standard Deviation
Standard deviation gauges how much the data points in a set vary from the mean of the set. In this scenario, it is essential to understand variability in both driving speeds and miles per gallon. It can tell us how spread out the driving speeds and fuel efficiencies are from typical values.
If the standard deviation is small, it means the data points are closely clustered around the mean. A large standard deviation indicates a wider spread. This measure is crucial when calculating the sample correlation coefficient because it normalizes the covariance:
  • Lower standard deviation implies less variability in data.
  • A higher standard deviation suggests more diverse data points.
  • It is a vital component in determining how reliable the linear relationship is.
By understanding how standard deviation affects data analysis, we can better interpret the stability of driving speed impacts on fuel efficiency.
Miles per Gallon
Miles per gallon (MPG) is a common measure of a car's fuel efficiency, indicating how far a vehicle can travel using one gallon of fuel. In this study, MPG is analyzed against different driving speeds to understand how speed influences fuel consumption.
MPG provides a tangible way for drivers to assess the efficiency of their driving habits. It helps in identifying any potential waste of fuel and allows for more economically sound driving practices. Comprehending the relationship between driving speed and MPG can guide drivers in making informed decisions to enhance efficiency, save money, and reduce their carbon footprint.
  • Higher MPG means better fuel efficiency.
  • Analyzing MPG against speed shows the impact of driving habits on fuel consumption.
  • Optimization of MPG can contribute to sustainable driving practices.
Attention to MPG can lead to improved economic and environmental outcomes.
Driving Speed
Driving speed affects not only the time it takes to reach your destination but also how your vehicle uses fuel. This concept is crucial when analyzing the data collected on speed and miles per gallon. Different speeds can influence how efficiently an engine operates and, consequently, its fuel consumption.
Understanding the impact of driving speed on MPG can lead to smarter driving strategies that optimize fuel use. It also highlights the balance between speed and efficiency, as driving too fast or too slow might not derive optimal results.
  • Driving at optimal speeds can improve MPG.
  • Extreme speeds, whether high or low, might negatively impact fuel efficiency.
  • Researching the effects of various speeds can assist in making driving more economical.
By examining driving speed, one can learn to make informed decisions about maintaining a balance between travel time and fuel consumption.

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Most popular questions from this chapter

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