Grade 8 Scatter Plots: Reading Scatter Plots

Worked examples

Real problems from the practice worksheets, solved one step at a time.

1. Example 1

   x = Age of car (years) · y = Value ($ thousands)

   Data: Age of car (years) (x) and Value ($ thousands) (y)

   x	2	3	4	5
   y	25	24	23	20
   x	6	7	8	9
   y	18	17	14	11

   Find the point where the age of the car is 5. What is the value of the car (in thousands of dollars) at that point?

   1. Find 5 on the x-axis (the age of the car).
   2. Go straight up to the point above it: (5, 20).
   3. Read across to the y-axis: the value of the car (in thousands of dollars) at that point is 20.

   Answer: 20

2. Example 2

   x = x · y = y

   Data: x (x) and y (y)

   x	2	3	4	5
   y	15	6	9	22
   x	6	7	8	9
   y	24	15	16	29

   What type of association does the scatter plot show between the x-values and the y-values?

   1. Read the points from left to right (x from 2 to 9). Their y-values are 15, 6, 9, 22, 24, 15, 16, 29.
   2. Overall they go up: from 15 on the left to 29 on the right. As x increases, y tends to increase.
   3. When both go up together, the association is positive.

   Answer: positive

3. Example 3

   x = Temperature (°F) · y = Cones sold

   Data: Temperature (°F) (x) and Cones sold (y)

   x	68	71	74	77
   y	71	86	89	104
   x	80	83	86	89
   y	113	116	131	134

   How many points show a number of cones sold greater than 114?

   1. List the y-value of each of the 8 points: 71, 86, 89, 104, 113, 116, 131, 134.
   2. Greater than 114: 116, 131, 134 (not 71, 86, 89, 104, 113).
   3. Count them: 3 points.

   Answer: 3