Understanding Psychology (10th Ed)

median
The point in a distribution of 
scores that divides the distribution 
exactly in half when the scores are 
listed in numerical order.
mode
The most frequently occurring 
score in a set of scores.
mean 
The average of all scores, 
arrived at by adding scores together 
and dividing by the number of scores.
central tendency 
An index of the 
central location within a distribution 
of scores; the most representative score 
in a distribution of scores (the mean, 
median, and mode are measures of 
central tendency).
feL82795_app_A2-A19.indd Page A-7 8/4/10 8:54 PM user-f465
feL82795_app_A2-A19.indd Page A-7 8/4/10 8:54 PM user-f465
/Users/user-f465/Desktop
/Users/user-f465/Desktop

---

A-8 Appendix 
Going by the Numbers: Statistics in Psychology
Comparing the Three M’s: Mean 
Versus Median Versus Mode
If a sample is suffi ciently large, there is generally little difference between the mean, 
median, and mode. The reason is that with large samples, scores typically form what 
is called a normal distribution. A normal distribution is a distribution of scores that 
produces a symmetrical, bell-shaped curve, such as the one displayed in Figure 3 in 
which the right half mirrors the left half and in which the mean, median, and mode 
all have the same value.
Most large distributions—those containing many scores—produce a normal 
curve. For instance, if you asked a large number of students how many hours a week 
they studied, you might expect to fi nd that most studied within a similar range of 
hours, and there would be a few who studied many, many hours and a very few 
who did not study at all. There would be many scores hovering around the center 
of the distribution of scores, then, and only a few at the extremes—producing a 
normal distribution. Many phenomena of interest to psychologists produce a normal 
curve when graphed. For example, the distribution of IQ scores among the general 
population falls into a normal distribution. 
The mean, median, and mode fall at exactly the same point in a normal distribu-
tion. This means that in a normal distribution of scores, the mean score will divide 
the distribution exactly in half (making it the median), and it will be the most fre-
quently occurring score in the distribution (making it the mode). 
The mean, median, and mode differ, however, when distributions are not normal. 
In cases in which the distributions are skewed, or not symmetrical, there is a “hump” at 
one end (see Figures 4 and 5). For instance, if we gave a calculus exam to a group of 
students enrolled in an elementary algebra class, we would expect that most of the 
students would fail the test, leading to low scores being overrepresented in the distribu-
tion as in Figure 4. On the other hand, if we gave the same students a test of elementary