What is the iqr (inter quartile range) for the data set?
11 6 19 14 21 7 13 15 15
8
9
15
17

You need to determine the mean and the quartiles first. Then, draw the box and whiskers plot.. See the guide below... 

1 
Determine the median of the data. List the numbers in order from smallest to largest and cross off pairs of numbers one at a time, one number from each side. If left with two numbers in the middle, average them by adding them together and dividing by two. 

Example: 

6, 17, 11, 13, 7, 14 

Reorganize and eliminate: 

6, 7, 11, 13, 14, 17 

7, 11, 13, 14 

11, 13 

Average: 

11 + 13 = 24 

24 / 2 = 12 

Median (or 2nd quartile) is 12. 

2 
Find the quartiles. Split the data in two by the median, then find the median of each half of the data. 

Example: 

6, 7, 11 

7 

1st (lower) quartile is 7. 

13, 14, 17 

14 

3rd (higher) quartile is 14. 

3 
Above your number line, draw a rectangle whose ends line up with your first and third quartile values. 

4 
Divide the rectangle, or box, in two with a line representing the value of the median. 

5 
Draw two points above the number line, one for your minimum value (6, in this example), and one with your maximum value (17, in this example). Connect the points with horizontal lines leading to the edges of your rectangle. These lines are the "whiskers" of the plot