8th grade · Math · Unit 6 of 17
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.
Arizona Academic Standards learning standard: 8.SP.A.1; 8.SP.A.2 · scatter plots and patterns of association
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 Arizona
In the Arizona Academic Standards, this is unit 6 of 17 in 8th grade Math, after Big, Small & Real Numbers and before Tessellations & Transformations.
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: Arizona Department of Education
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
On a scatter plot, as the x-values increase, the y-values go down. The points are loosely scattered around a line. How would you describe the relationship?
- weak negative
- strong negative
- strong positive
- weak positive
weak 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 2
Which graph is best for showing how height and arm span are related for 30 students?
- a scatter plot
- a pictograph
- 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 3
Why should you investigate an outlier before removing it?
- Outliers are never important.
- It might be an error, or it might be an important true value.
- Removing it makes the data more honest.
- Outliers are always mistakes.
It might be an error, or it might be an important true value.
Hint: An outlier can come from a measuring mistake or a real unusual case. Find out which.
Question 4
Which question needs two-variable data?
- What is each student's favorite lunch?
- Do students who sleep more hours get higher test scores?
- What color is each car in the parking lot?
Do students who sleep more hours get higher test scores?
Hint: One-variable data describes one thing for each person or object. Two-variable data pairs two measurements to look for a relationship.
Question 5
Which variable is continuous?
- the number of plants on a shelf
- the number of flowers that open
- the height of a plant
- the number of leaves on a plant
the height of a plant
Hint: Heights can be measured to any precision, so they are continuous.
Question 6
The points on a scatter plot are spread out with no pattern. How would you describe the relationship?
- none
- strong negative
- weak positive
- strong positive
none
Hint: If one variable does not predict the other, there is no relationship.