Download Domain: Statistics and Probability (SP) Cluster: Investigate patterns

Domain: Statistics and Probability (SP)
Cluster: Investigate patterns of association in bivariate data.
Standard: 8.SP.1. Construct and interpret scatter plots for bivariate measurement data to
investigate patterns of association between two quantities. Describe patterns such as clustering,
outliers, positive or negative association, linear association, and nonlinear association.
Standards for Mathematical Practice (MP):
MP.2. Reason abstractly and quantitatively.
MP.4. Model with mathematics.
MP.5. Use appropriate tools strategically.
MP.6. Attend to precision.
MP.7. Look for and make use of structure.
Connections:
This Cluster is connected to the grade 8 Critical Area of Focus #1, Formulating and reasoning
about expressions and equations, including modeling an association in bivariate data
with a linear equation, and solving linear equations and systems of linear equations.
Explanations and Examples
8.SP.1 Bivariate data refers to two variable data, one to be graphed on the x-axis and the other
on the y-axis. Students represent measurement (numerical) data on a scatter plot, recognizing
patterns of association. These patterns may be linear (positive, negative or no association) or
non-linear
Students build on their previous knowledge of scatter plots examine relationships between
variables. They analyze scatterplots to determine positive and negative associations, the degree
of association, and type of association. Students examine outliers to determine if data points are
valid or represent a recording or measurement error. Students can use tools such as those at the
National Center for Educational Statistics to create a graph or generate data sets.
(http://nces.ed.gov/nceskids/createagraph/default.aspx)
Examples:
 Data for 10 students‘ Math and Science scores are provided in the chart. Describe the
association between the Math and Science scores.
Student
Math
Science

1
64
68
2
50
70
3
85
83
4
34
33
5
56
60
6
24
27
7
72
74
8
63
63
9
42
40
10
93
96
Data for 10 students‘ Math scores and the distance they live from school are provided in
the table below. Describe the association between the Math scores and the distance
they live from school.
Student
Math
score
Dist from
school
(miles)
1
2
3
4
5
6
7
8
9
10
64
50
85
34
56
24
72
63
42
93
0.5
1.8
1
2.3 3.4 0.2 2.5 1.6 0.8 2.5
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
Data from a local fast food restaurant is provided showing the number of staff members
and the average time for filling an order are provided in the table below. Describe the
association between the number of staff and the average time for filling an order.
Number of staff
Average time to fill order
(seconds)

3
180
4
138
5
120
6
108
7
96
8
84
The chart below lists the life expectancy in years for people in the United States every
five years from 1970 to 2005. What would you expect the life expectancy of a person in
the United States to be in 2010, 2015, and 2020 based upon this data? Explain how you
determined your values.
Date
1970 1975 1980 1985 1990 1995 2000 2005
Life Expectancy (in years) 70.8 72.6 73.7 74.7 75.4 75.8 76.8 77.4
Instructional Strategies
Building on the study of statistics using univariate data in Grades 6 and 7, students are now ready
to study bivariate data. Students will extend their descriptions and understanding of variation to
the graphical displays of bivariate data.
Scatter plots are the most common form of displaying bivariate data in Grade 8. Provide scatter
plots and have students practice informally finding the line of best fit. Students should create and
interpret scatter plots, focusing on outliers, positive or negative association, linearity or curvature.
By changing the data slightly, students can have a rich discussion about the effects of the change
on the graph. Have students use a graphing calculator to determine a linear regression and
discuss how this relates to the graph. Students should informally draw a line of best fit for a
scatter plot and informally measure the strength of fit. Discussion should include ―What does it
mean to be above the line, below the line?‖
The study of the line of best fit ties directly to the algebraic study of slope and intercept. Students
should interpret the slope and intercept of the line of best fit in the context of the data. Then
students can make predictions based on the line of best fit.
Common Misconceptions:
Students may believe:
Bivariate data is only displayed in scatter plots. 8.SP.4 in this cluster provides the opportunity to
display bivariate, categorical data in a table.
In general, students think there is only one correct answer in mathematics. Students may
mistakenly think their lines of best fit for the same set of data will be exactly the same. Because
students are informally drawing lines of best fit, the lines will vary slightly. To obtain the exact
line of best fit, students would use technology to find the line of regression.
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8.SP.1
Domain: Statistics and Probability
Cluster: Investigate patterns of association in bivariate data.
Standard: 8.SP.2. Know that straight lines are widely used to model relationships between two
quantitative variables. For scatter plots that suggest a linear association, informally fit a straight
line, and informally assess the model fit by judging the closeness of the data points to the line.
Standards for Mathematical Practice (MP):
MP.2. Reason abstractly and quantitatively.
MP.4. Model with mathematics.
MP.5. Use appropriate tools strategically.
MP.6. Attend to precision.
MP.7. Look for and make use of structure.
Connections:
See 8.SP.1.
Explanations and Examples
8.SP.2 Students understand that a straight line can represent a scatter plot with linear
association. The most appropriate linear model is the line that comes closest to most data points.
The use of linear regression is not expected.
Examples:
 The capacity of the fuel tank in a car is 13.5 gallons. The table below shows the number
of miles traveled and how many gallons of gas are left in the tank. Describe the
relationship between the variables. If the data is linear, determine a line of best fit. Do
you think the line represents a good fit for the data set? Why or why not? What is the
average fuel efficiency of the car in miles per gallon?
Miles
0
Traveled
Gallons
0
Used
75
120
160
250
300
2.3
4.5
5.7
9.7
10.7
Instructional Strategies
See 8.SP.1.
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8.SP.2
Domain: Statistics and Probability
Cluster: Investigate patterns of association in bivariate data.
Standard: 8.SP.3. Use the equation of a linear model to solve problems in the context of bivariate
measurement data, interpreting the slope and intercept. For example, in a linear model for a biology
experiment, interpret a slope of 1.5 cm/hr as meaning that an additional hour of sunlight each day is
associated with an additional 1.5 cm in mature plant height.
Standards for Mathematical Practice (MP):
MP.2. Reason abstractly and quantitatively.
MP.4. Model with mathematics.
MP.5. Use appropriate tools strategically.
MP.6. Attend to precision.
MP.7. Look for and make use of structure.
Connections:
See 8.SP.1.
Explanations and Examples
8.SP.3 Linear models can be represented with a linear equation. Students interpret the slope and yintercept of the line in the context of the problem.
Examples:
 1. Given data from students‘ math scores and absences, make a scatterplot.

2. Draw a line of best fit, paying attention to the closeness of the data points on either side of the line.

3. From the line of best fit, determine an approximate linear equation that models the given data
(about y =  25 x  95 )

4. Students should recognize that 95 represents the y intercept and
3

25 represents
3
the slope of the line.
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5. Students can use this linear model to solve problems. For example, through substitution, they can use
the equation to determine that a student with 4 absences should expect to receive a math score of about
62. They can then compare this value to their line.
Instructional Strategies
See 8.SP.1.
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8.SP.3
Domain: Statistics and Probability
Cluster: Investigate patterns of association in bivariate data.
Standard: 8.SP.4. Understand that patterns of association can also be seen in bivariate categorical data
by displaying frequencies and relative frequencies in a two-way table. Construct and interpret a two-way
table summarizing data on two categorical variables collected from the same subjects. Use relative
frequencies calculated for rows or columns to describe possible association between the two variables. For
example, collect data from students in your class on whether or not they have a curfew on school nights
and whether or not they have assigned chores at home. Is there evidence that those who have a curfew
also tend to have chores?
Standards for Mathematical Practice (MP):
MP.2. Reason abstractly and quantitatively.
MP.3. Construct viable arguments and critique the reasoning of others.
MP.4. Model with mathematics.
MP.5. Use appropriate tools strategically.
MP.6. Attend to precision.
MP.7. Look for and make use of structure.
Connections:
See 8.SP.1.
Explanations and Examples
8.SP.4 Students recognize that categorical data can also be described numerically through the
use of a two-way table. A two-way table is a table that shows categorical data classified in two
different ways. The frequency of the occurrences are used to identify possible associations
between the variables. For example, a survey was conducted to determine if boys eat breakfast
more often than girls. The following table shows the results:
Male
Female
Eat breakfast on a
190
110
regular basis
Do not eat breakfast
130
165
on a regular basis
Students can use the information from the table to compare the probabilities of males eating
breakfast (190 of the 320 males à 59%) and females eating breakfast (110 of the 375 females
à29%) to answer the question. From this data, it can be determined that males do eat breakfast
more regularly than females.
Example:
 The table illustrates the results when 100 students were asked the survey questions: Do
you have a curfew? and Do you have assigned chores? Is there evidence that those who
have a curfew also tend to have chores?
Solution: Of the students who answered that they had a curfew, 40 had chores and 10 did not. Of the
students who answered they did not have a curfew, 10 had chores and 40 did not. From this sample, there
appears to be a positive correlation between having a curfew and having chores.
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8.SP.4