6. Students should be able to draw and to understand figures/graphs that display several different
means (or medians) representing different treatment conditions or different groups.
Line graphs are typically used when the treatment conditions or groups correspond to
numerical values (interval or ratio scales). Otherwise, bar graphs are used to display the
Other Lecture Suggestions
1. The mean can be introduced as the “average” that students already know how to calculate.
For example, ask students to find the average telephone bill if one month is $20 and the next
month is $30. Everyone gets this right. Now, add a third month with a bill of $100. Many
students will tell you that the mean is now $75. This is a good chance to introduce the formula
for the mean, showing that the total cost (ΣX = $150) is divided over three (n = 3) months.
2. Sketch a simple histogram and label the values along the X axis using 1, 2, 3, 4, and 5. Ask
the students what will happen to the distribution if you add 10 points to every score. Answer:
The whole distribution moves 10 points to the right. (You can keep the same sketch, simply re-
label the values on the X axis to 11, 12, 13, 14, and 15.) Note that the mean (middle) has shifted
10 points to the right.
Go back to the original distribution and ask what would happen if every score were
multiplied by 10. This time the 1s become 10s, the 2s become 20s, and so on. Again, you can
keep the same sketch, simply re-label the values on the X axis to 10, 20, 30, 40, and 50. After
multiplying, the mean (that used to be at 3) is located at 30 (10 times bigger).
3. The idea that the mean is not always a central, representative value can be demonstrated by
starting with a simple, symmetrical distribution consisting of scores 1, 2, 3, 4, and 5 with
frequencies of 1, 2, 4, 2, and 1, respectively. Just looking at the frequency distribution
histogram, it is easy to see that the mean is 3, but you can also demonstrate that the n = 10 scores
add up to ΣX = 30.
Now, move the score at X = 5 to a new location at X = 55 and have students find the new
mean (adding 50 points to one score adds 50 points to the total, so ΣX is now 80 and the mean is
8). Note that the new mean, 8, is not a representative value. In fact, none of the scores is located
around X = 8. Finally, you can introduce the median using the original distribution (Median = 3)
and then see what happens to this measure of central tendency when X = 5 is moved to X = 55
(the median is still 3). The median is relatively unaffected by extreme scores.
Instructor Notes – Chapter 3 – page 34