How to explain it
At this standard, students recognize when a linear association supports a line of best fit, describe correct placement through the center of the data cloud, and assess model fit by judging how tightly data points cluster around the line.
The anchor students hold onto: A line of best fit is a straight line drawn through the middle of a linear scatter plot, minimizing overall distance to all points. Judge fit by how tightly points cluster around it.
In Scatter Plot Slope & Intercept (8.SP.A.3), students extend this skill by interpreting the slope and y-intercept of the line of best fit in real-world context.
Worked examples
Example 1
Recognize Linear Association
Should you fit a line here?
Step 1Check direction: do points rise or fall consistently?
Step 2If yes (linear trend), a line of best fit is appropriate.
Step 3If the plot is curved or random, no line is needed.
AnswerYes — a consistent linear trend supports a line of best fit.
Example 2
Fit the Line
Where should the trend line go?
Step 1Draw ONE straight line through the center of the cloud.
Step 2Balance: roughly half the points above and half below.
Step 3The line does not need to pass through any data point.
AnswerThrough the middle — about half the points above, half below.
Example 3
Assess Model Fit
Is this a strong or weak fit?
Step 1Examine how closely points cluster around the trend line.
Step 2Tight cluster (small gaps) → strong model fit.
Step 3Loose scatter (large gaps) → weak model fit.
AnswerStrong fit: tight cluster. Weak fit: points spread far from the line.
Common mistakes
What students write
Drawing the line through only the two extreme points (leftmost and rightmost).
The fix
The line must balance ALL points — about half above and half below the entire cloud, not just the two endpoints.
Try this
A student analyzed the scatter plot shown. Here is their work: I drew my trend line by connecting the leftmost point (2, 2) and the rightmost point (9, 6). Since my line passes through two real data points, it must be the best fit. Any line through two actual data points is automatically the line of best fit. Find the error in the student's reasoning. Then describe how to correctly draw the line of best fit.
What students write
Thinking a line fits better when it passes through more data points exactly.
The fix
Fit is judged by how tightly ALL points cluster around the line, not how many the line touches exactly.
Teacher tip
Head off the two predictable errors before they happen. First: The line must balance ALL points — about half above and half below the entire cloud, not just the two endpoints. Second: Fit is judged by how tightly ALL points cluster around the line, not how many the line touches exactly.