Quartiles + Box and Whisker Plots

Quartiles Step 1: Find the Median. This is called Q2, or

the second quartile.

Step 2: Split the first half into 2 equal groups (same rules as finding the median). The middle value is called Q1, or the first quartile.

Step 3: Find median of top half of numbers (Q3).

You have split the numbers into 4 quarters.

12, 15, 18, 24, 26, 29, 30, 32

Q2 = average of 24 and 26Q2 = 25

12, 15, 18, 24, 26, 29, 30, 32

Q2 = 25

Q1 = 16.5

Q3 = 29.5

Box and Whisker Plot

1) Draw a number line from the lowest to highest value

2) Draw a line above Q1, Q2 and Q3

10 12 14 16 18 20 22 24 26 28 30 32

Box and Whisker Plot

2)Draw a line above Q1, Q2 and Q3

3) Attach lines to make the “box”

10 12 14 16 18 20 22 24 26 28 30 32

Box and Whisker Plot

2) Draw a line above Q1, Q2 and Q3

3) Attach lines to make the “box”

10 12 14 16 18 20 22 24 26 28 30 32

Box and Whisker Plot

4) Draw a line over the minimum and maximum values

5) Draw “whiskers”

10 12 14 16 18 20 22 24 26 28 30 32

Example

6, 8, 12, 14, 17, 19, 21, 24, 25

Q2

Q1 = 10 Q3 = 22.5

Measures of Dispersion

Variation Interval: [Xmin , Xmax]

100% of data in this interval

Range: Xmax – Xmin

Interquartile Interval: [Q1, Q3]

50% of data in this interval

Interquartile Range: Q3 – Q1