Grade 9 · Math · Unit 12 of 15
Data & Scatter Plots
Quartiles, box plots and correlation
What your child practises
Finding the median and quartiles of a data set, reading a box plot, telling positive, negative and no correlation apart, and using a line of best fit to make predictions. Correlation does not prove one thing causes another.
Ontario Curriculum expectation: D1.1, D1.2, D1.3 · quartiles and box plots, scatter plots, correlation and lines of best fit
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
Eight students recorded the minutes they read last night: 23, 17, 29, 11, 27, 39, 15, 35. What is the median?
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25
Hint: Put the values in order. With 8 values, the median is the mean of the 4th and 5th: (23 + 27) ÷ 2 = 25.
Question 2
A line of best fit for hours of practice (x) and score (y) is y = 3x + 8. What score does the model predict after 4 hours?
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20
Hint: Substitute x = 4: y = 3 × 4 + 8 = 20.
Question 3
A line of best fit is used to…
- Join every point exactly
- Make the data larger
- Prove one variable causes the other
- Describe the trend and predict values
Show answer
Describe the trend and predict values
Hint: The line models the overall trend. It need not pass through every point.
Question 4
Shoe size and test marks for a class are plotted. The points look scattered with no pattern. What does that suggest?
- A strong positive correlation
- There is little or no correlation
- A strong negative correlation
- The data must be wrong
Show answer
There is little or no correlation
Hint: Without a visible trend in the points, the variables are not related.
Question 5
Eight students recorded the minutes they read last night: 50, 48, 42, 32, 56, 36, 46, 38. What is the first quartile, Q1?
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37
Hint: Q1 is the median of the lower half (32, 36, 38, 42): (36 + 38) ÷ 2 = 37.
Question 6
Eight students recorded the minutes they read last night: 49, 43, 53, 33, 41, 37, 59, 27. What is the interquartile range (Q3 − Q1)?
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16
Hint: Q1 = 35 and Q3 = 51 (the medians of the lower and upper halves). Subtract: 51 − 35 = 16.