When we look at a single variable, the first thing we usually want to know is: What is the average value? But the term “average” can have different meanings. Usually, we think of the arithmetic average, or mean. But the mean can only be calculated only if the data are interval level—when they have both order and a scale. In addition, sometimes, if there are outliers in the data, the mean is skewed and so doesn’t really reflect the central tendency of the data. A second possible way of measuring the “average” of an attribute is to calculate the median. This is the attribute held by the case in the center of the distribution. The median is reported either if the data are ordinal level (they have order, but the unit is not of a constant size) or if they are interval level but skewed. A final measure of central tendency is the mode. The mode is the category of your variable with the most cases. The mode is the only measure of central tendency which that can be reported for nominal-level data (where the attributes can only be categorized, not ordered), but it can also be reported for interval- and ordinal-level data.
After reading this chapter, you should:
- Be able to calculate the mean, median, and mode from raw data
- Be able to calculate the mean, median, and mode from tabular data
- Be able to choose the appropriate measure of central tendency depending on the level of measurement of a variable
- Know how to get measures of central tendency in SPSS