These are ways to characterize a population or a sample. The *mean* is the average score. It is calculated by dividing the sum of scores by the number of scores. For example, the average of the series {4, 7, 8, 9, 9} equals 7.4, (4 7 8 9 9 divided by 5). The *median* refers to the number that falls in the middle of the sample; half of the scores lie above it and half lie below. In this case the median is 8. The *mode* refers to the most common score. In this case the mode is 9. Each measure of central tendency has different advantages and disadvantages.