Measures of central tendency are the central or middle or average values. These are mean, median and mode.
Mean (popularly called arithmetic mean) is the sum of all observations divided by number of observations.
Median is the middle value in terms of location when the observations are sorted in ascending or descending order.
Mode is the most repeated observation and usually lies in the center of distribution in case of normally distributed data
Measures of dispersion are the values that show the spread-ness of the data. The most commonly used measures of dispersion are: range and standard deviation.
Range shows how much the observation is spread in terms of extreme values. So, range is the highest value minus lowest value.
Standard deviation shows the dispersion of observation from the mean. It is the root mean squared deviation of individual observations from the mean.
Steps to calculate measures of central tendency and dispersion using SPSS
Let’s work on sample data in SPSS (e.g. demo.sav from your installation directory) or any data of your own. We can do by any one of the following two methods.
Step 1. Identify the numerical variable (such as age, height, weight, income etc.) which is entered in number form and whose calculation of measures of central tendency and dispersion is meaningful.