Basic Statistical Concepts
A. Measures of Central Tendency: Mean, Median, and Mode
1. Mean
The mean, or average, is a number calculated by adding a series of values and then
dividing by the number of values. For example, to find the mean of the numbers 5, 19,
and 27, we add them and divide by the number of values (3). The mean would then be
2. Mode
The mode is the single most common value in a series of scores. For example, if we
were looking at the following scores on a ten-point quiz: 10, 10, 9, 9, 8, 8, 7, 7, 7, 6, 5,
we would determine the mode by observing which score occurred most frequently. Now,
3. Median
The median is the midpoint or number that divides a series of values (which are ranked
in ascending or descending order). For the quiz discussed above, the median is 8
A percentage is a portion based on 100. Use of percentages allows one to compare
groups of different sizes.
Example: Comparing Populations of Different Sizes
If we are comparing contributors to a town's Baptist and Roman Catholic
churches, the absolute numbers of contributors could be misleading if there
were many more Baptists than Catholics living in the town. With
percentages, we can obtain a more meaningful comparison, showing the
proportion of persons in each group who contribute to their respective
C. Statistical Control vs. Control Groups