1. Scatter Plots and Equations of Lines
ALGEBRA 1 LESSON 6-6
(For help, go to Lessons 1-9.)
Use the data in each table to draw a scatter plot. 1 2 3 4 –3 8 9 5
–25 y x 1. 21 15 12 7 2.
6-6

2. Scatter Plots and Equations of Lines
ALGEBRA 1 LESSON 6-6
Solutions
1. Graph the points:  
(1, 2)
(2, –3)
(3, 8)
(4, 9)
(5, –25)
2. Graph the points:  
(1, 21)
(2, 15)
(3, 12)
(4, 9)
(5, 7)
6-6

3. Scatter Plots and Equations of Lines
ALGEBRA 1 LESSON 6-6
Make a scatter plot to represent the data. Draw a trend line and write an equation for the trend line. Use the equation to predict the time needed to travel 32 miles on a bicycle.
Speed on a Bicycle Trip
Step 1  Draw a scatter plot. Use a      
straight edge to draw a
trend line. Estimate two
points on the line.
Miles
Time
(min) 5 27 10 46 14 71 18 78 22
107
6-6

4. Scatter Plots and Equations of Lines
ALGEBRA 1 LESSON 6-6
(continued)
Step 2  Write an equation of the trend line.
m = = =
y2 – y1
x2 – x1
107 – 27
22 – 5 80 17
y – y1 = m(x – x1)   Use point-slope for m.
y – 27 = 5(x – 5) Substitute 5 for m and (5, 27) for (x1, y1).
Step 3  Predict the time needed to travel 32 miles.
y – 27 = 5(32 – 5) Substitute 32 for x.
y – 27 = 5(27) Simplify within the parentheses.
y – 27 =135 Multiply.
y = Add 27 to each side.
The time needed to travel 32 miles is about 162 minutes.
6-6

5. 28/07/10 Scatter Graphs What is a scatter graph? The line of best fit
Correlation

6. What is a scatter graph?
Scatter graphs are used to compare two sets of data (e.g. age and height).
They can be used to look for a connection between the two sets of data.
This connection is called a correlation.

7. The line of best fit shows the general trend of the data.
28/07/10
The Line of Best Fit
When there is a connection between two sets of data, we can draw a line of best fit.
The line of best fit shows the general trend of the data.
You can draw the line wherever you think it fits best..
...but some lines are better than others!

8. Line of Best Fit ? 1 2 3 4 5

9. Do you agree or disagree with each statement?
Lines of Best Fit
Do you agree or disagree with each statement?
The line should go through all of the points.
It is always easy to see where the line should go.
The line must go from bottom left to top right.
There should be about the same number of points above the line as below it.
There is always only one correct line for each graph.
The line should go through as many points as possible.
The line must go through zero.

10. Do you agree or disagree with each statement?
Lines of Best Fit
Do you agree or disagree with each statement?
The less 'scatter' there is about the line, the stronger the relationship is between the two sets of data.
A line going up (from left to right) shows that as one quantity increases, so does the other.
If there are more points on the graph, it is harder to tell whether or not there is a correlation.

11. Line of best fit slopes upwards. No correlation
Types of Correlation
Positive correlation
Line of best fit slopes upwards.
No correlation
Cannot draw a line of best fit.
Negative correlation
Line of best fit slopes downwards.
Weak correlation
Difficult to draw a line of best fit
Strong correlation
Points lie close to the line of best fit

12. Scatter Plots and Equations of Lines
ALGEBRA 1 LESSON 6-6
1996
5086.6
1997
1998
1999
4930.0
4619.3
4266.8
Year
No. of Crimes
1995
5275.9
[Source: Crime in the United States,
1999, FBI, Uniform Crime Reports]
Use a graphing calculator to find the equation of the line of best fit for the data below. What is the correlation coefficient?
U.S. Crime Rate
(per 100,000 inhabitants)
6-6

13. Scatter Plots and Equations of Lines
ALGEBRA 1 LESSON 6-6
(continued)
Step 1  Use Desmos table feature on the
graphing calculator. Enter the data for
years and then enter data for crimes.
Step 2  Use the y1~mx1+b to create an equation
for best fit line.
The equation for the line of best fit is y = –248.55x for values a and b rounded to the nearest hundredth. The value of the correlation coefficient is –
6-6

14. Scatter Plots and Equations of Lines
ALGEBRA 1 LESSON 6-6
1. Graph the data in a scatter plot.
Draw a trend line.
2. Write an equation for the trend
line.
3. Predict the number of households
in the U.S. in (Use 105 for x.)
4. Use a calculator to find the line of
best fit for the data.
5. What is the correlation coefficient?
Number of Households in the U.S.
[Source: U.S. Census Bureau, Current Population Reports.
From Statistical Abstract of the United States, 2000]
1980
80.8
1985
1990
1995
86.8
93.3
99.0
Year
Households
(millions)
1975
71.1
y – 86.8 = 1.3(x – 1985)
about million households
y = 1.366x –
0.9944
6-6