3.1.4 • Interpreting the Slope and Vertical Intercept of a Scatter Plot
Activity
Here are several scatter plots.
Use scatter plot A to answer questions 1 – 2.
Scatter plot A:
Using the horizontal axis for and the vertical axis for , interpret the slope of the linear model in the situation shown.
Compare your answer: Your answer may vary, but here is a sample.
For every increase in age of one year, the linear model estimates that the reaction time decreases by about 9.25 milliseconds.
If the linear relationship continues to hold for the situation, interpret the -intercept of the linear model.
Compare your answer: Your answer may vary, but here is a sample.
The reaction time estimated by the linear model for a newborn is 400 milliseconds.
Use scatter plot B to answer questions 3 and 4.
Scatter plot B:
Using the horizontal axis for and the vertical axis for , interpret the slope of the linear model in the situation shown.
Compare your answer: Your answer may vary, but here is a sample.
For every additional banana, the linear model estimates that the price increases by about $0.44.
If the linear relationship continues to hold for the situation, interpret the -intercept of the linear model.
Compare your answer: Your answer may vary, but here is a sample.
The price estimated by the linear model for purchasing zero bananas is $0.04.
Use scatter plot C to answer questions 5 – 6.
Scatter plot C:
Using the horizontal axis for and the vertical axis for , interpret the slope of the linear model in the situation shown.
Compare your answer: Your answer may vary, but here is a sample.
For each additional square foot, the linear model estimates that the cost to install flooring increases by about $4.
If the linear relationship continues to hold for the situation, interpret the -intercept of the linear model.
Compare your answer: Your answer may vary, but here is a sample.
The fixed cost to install flooring is $87.
Use scatter plot D to answer questions 7 and 8.
Scatter plot D:
Using the horizontal axis for and the vertical axis for , interpret the slope of the linear model in the situation shown.
Compare your answer: Your answer may vary, but here is a sample.
For every increase in temperature of 1 degree Celsius, the linear model estimates that the volume decreases by about 2.4 cubic centimeters.
If the linear relationship continues to hold for the situation, interpret the -intercept of the linear model.
Compare your answer: Your answer may vary, but here is a sample.
25 cubic centimeters is the volume estimated by the linear model for a temperature of zero degrees Celsius.
Are you ready for more?
Extending Your Thinking
Clare, Diego, and Elena collect data on the mass and fuel economy of cars at different dealerships.
Clare finds the line of best fit for data she collected for 12 used cars at a used car dealership. The line of best fit is where is the car’s mass, in kilograms, and is the fuel economy, in miles per gallon.
Diego made a scatter plot for the data he collected for 10 new cars at a different dealership.
Elena made a table for data she collected on 11 hybrid cars at another dealership.
| mass (kilograms) | fuel economy (miles per gallon) |
|---|---|
| 1,100 | 38 |
| 1,200 | 39 |
| 1,250 | 35 |
| 1,300 | 36 |
| 1,400 | 31 |
| 1,600 | 27 |
| 1,650 | 28 |
Interpret the slope and -intercept of Clare’s line of best fit in this situation.
Compare your answer:
The slope means that the fuel economy decreases 9 miles per gallon for every 1,000 kg increase in mass. The -intercept represents the fuel economy of a car, in miles per gallon, that has a mass of 0 kg.
Diego looks at the data for new cars and used cars. He claims that the fuel economy of new cars decreases as the mass increases. He also claims that the fuel economy of used cars increases as the mass increases. Do you agree with Diego’s claims? Be prepared to show your reasoning.
Compare your answer:
Diego’s claim about new cars is correct. You can look at the graph and see that the fuel economy tends to decrease as the mass increases because the scatter plot appears to have a negative slope. Diego’s claim about used cars is not correct because the line of best fit has a negative slope, which means that the fuel economy decreases as the mass increases.
Elena looks at the data for hybrid cars and correctly claims that the fuel economy decreases as the mass increases. How could Elena compare the decrease in fuel economy as mass increases for hybrid cars to the decrease in fuel economy as mass increases for new cars? Be prepared to show your reasoning.
Compare your answer:
Elena needs to compare the slope of the data for hybrids to the slope of the data for new cars. She could do this by looking at the scatter plots and graphing an approximated line of best for each scatter plot. She could then see which line of best fit looks like it is decreasing faster or find and compare the slopes of each approximated line of best fit.
Self Check
Additional Resources
Using Linear Models
The Monopoly board game is popular in many countries. The scatter plot below shows the distance from “Go” to a property (in number of spaces moving from “Go” in a clockwise direction) and the price of the properties on the Monopoly board. The equation of the line is , where represents the price (in Monopoly dollars) and represents the distance (in number of spaces).
The model shows that as the distance from “Go” increases, the price of the property increases.
The slope of the equation of the linear model is 8. This means that for every space from “Go,” the price of the property increases $8.
Notice this gives an estimate. The most expensive property is 39 spaces from “Go” and costs $400. The model would give the price to be or $352.
The model gives a difference of $48 between the estimated price using the model and the actual price.
Try it
Using Linear Models
Looking at the linear model above, what claim can be made?
Compare your answer:
Here is one claim that can be made looking at the scatter plot and linear model:
As the mean July temperature of a city increases, so does the mean rainfall per year.