Question 39·Hard·Two-Variable Data: Models and Scatterplots
The scatterplot above shows the relationship between practice time and quiz score for 9 students. A line of best fit is drawn and is labeled with the equation .
The point labeled P was recorded incorrectly and should be removed. If P is removed and a new line of best fit is drawn for the remaining points, which choice is most likely the equation of the new line of best fit?
When an outlier is removed, think of the best-fit line as a balance point that can pivot. A point that is far to the right has strong leverage on the slope: if it lies below the trend, it pulls the slope down (flattens the line), so removing it makes the slope increase. Then use one or two representative x-values from the cluster to test which candidate equation gives reasonable predicted y-values.
Hints
Locate point P relative to the main cluster
Is P above or below the general trend of the other points, and is it far left or far right?
Think about how a far-right point affects slope
If a point is far to the right and below the trend, would it make the best-fit line flatter or steeper?
Compare what each equation predicts at two x-values
Use a small (like ) and a larger (like ) to see which line stays closest to the remaining points.
Desmos Guide
Enter the points except P in a table
In Desmos, create a table and enter the 8 points that are not labeled P (use the x-values as the first column and y-values as the second column).
Graph each candidate line
In separate expressions, enter the four equations from the answer choices (for example, y=3.2x+54, y=2.4x+56, and so on).
See which line stays closest to the remaining points
Visually compare which line runs through the middle of the 8 plotted points (with about as many points above as below). The line that best matches this is the one you should select.
Step-by-step Explanation
Use the scatterplot to identify the outlier’s position
From the graph, the points from about to follow an upward trend, but point P is at a large -value and has a much smaller -value than nearby points.
Decide how removing P changes the line of best fit
Because P is to the right of the other points and below the trend, it pulls the right side of the line downward.
Removing P would therefore make the new line of best fit steeper (it would have a larger slope than ).
Choose the equation that best matches the remaining points
A line that fits the remaining cluster should still predict a score in the high 50s when is around , and a score near when is around .
Among the choices, has a larger slope than and predicts
- at : (close to the left-side points)
- at : (close to the upper-right points)
So the most likely new line of best fit is .