Back-to-back stem-and-leaf plots

These are used to compare two similar sets of numerical data such as:

• two tests with the same total marks given to the same class
• the heights of boys and girls in a group of students
• the daily maximum temperatures for two separate months
• the ages of people attending two theatre productions.

Example 2

A class of 25 students sit for two Mathematics tests, each out of 100. Their results are recorded in the following back-to-back stem-and-leaf plot.

Test 1		Test 2
5 5	4
9 6 6 4	5	5 6 6
9 8 8 7 6 5 5 4 3 3	6	5 7
8 8 5 2 2 2 1	7	5 9
1	8	0 0 4 4 5 5 5 6 7 9 9
0	9	0 0 1 3 5 7 7	8\(\mid\)4 represents 84

Question	Solution
How many students scored 70 or more for each test?	Nine students in test 1 and 20 students in test 2 scored 70 or more.
How many students scored less than 50 for each test?	Two students in test 1 and zero in test 2 scored 50 or less.