8th grade · Math · Unit 8 of 15
Scatter Plots & Stories
Two-variable data and misleading graphs
What your child practices
Deciding when two-variable data is needed, picking the best graph, describing scatter plots as strong, weak, positive, negative or none, spotting outliers, and noticing graphs that mislead.
Texas Essential Knowledge and Skills (TEKS) teks student expectation: 8.5(C); 8.5(D); 8.11(A) · scatterplots, association and trend lines
How we explain it
Children can open this short lesson before they practice.
- A scatter plot shows two sets of data as points.
- Points that rise together show a positive relationship.
- A point far from the pattern is an outlier.
Example: As study time goes up, test scores go up. What kind of relationship is that? Both values rise together. That is a positive relationship. Answer: Positive
Scatter Plots & Stories in Texas
In the Texas Essential Knowledge and Skills (TEKS), this is unit 8 of 15 in 8th grade Math, after Functions and before Cones, Cylinders & Spheres.
Gradelings describes a child's practice on this unit as Beginning, Approaching, Meeting or Exceeding (Four practice steps). That reflects practice, not a report-card mark: the teacher decides the mark.
Source: Texas Education Agency (TEA)
Sample questions
A taste of this unit. In the app, questions change every time and get easier or harder to match your child.
Question 1
A data set is collected with a table of values. What goes in each row of a table for two-variable data?
- a pair of values, one for each variable
- only the largest value
- the mean of the data
- a single value for both variables
a pair of values, one for each variable
Hint: Each row records one pair, like a student's height and arm span.
Question 2
On a scatter plot, as the x-values increase, the y-values go down. The points lie very close to a line. How would you describe the relationship?
- strong positive
- weak negative
- weak positive
- strong negative
strong negative
Hint: Positive means both go up together; negative means one goes up as the other goes down. Strong means the points are close to a line.
Question 3
A graph has no title and no labels on its axes. What is the problem?
- The data must be wrong.
- The graph must be a scatter plot.
- Readers cannot tell what the data shows.
- Nothing, because the shape tells the story.
Readers cannot tell what the data shows.
Hint: A good graph has a title, labels, units and a source.
Question 4
The points on a scatter plot are spread out with no pattern. How would you describe the relationship?
- strong positive
- strong negative
- weak positive
- none
none
Hint: If one variable does not predict the other, there is no relationship.
Question 5
Which graph is best for showing how height and arm span are related for 30 students?
- a pictograph
- a scatter plot
- a bar graph of the heights only
- a circle graph
a scatter plot
Hint: A scatter plot shows two numerical variables as points, so you can look for a relationship.
Question 6
Which question needs two-variable data?
- How does the age of a car relate to its resale price?
- How many students ride the bus each day?
- What color is each car in the parking lot?
How does the age of a car relate to its resale price?
Hint: One-variable data describes one thing for each person or object. Two-variable data pairs two measurements to look for a relationship.