Mean difference is a mathematical term. You will also commonly hear it referred to as an average. It represents a generalized central value of a data set  for example, the central value of a series of numbers.
Calculating Mean Difference

Add up the numbers, or value sets, in your series and then divide that sum by the number of value sets. For example, if you are calculating the average number of eggs your hen lays in five days, track the number of eggs she lays on each day: Monday, 2 eggs; Tuesday, 1 egg; Wednesday, 3 eggs; Thursday, 2 eggs; Friday, 2 eggs. The total number of eggs is 10. Divide 10 by the number of days you tracked her laying, which is 5. 10 eggs divided by 5 days equals a mean or average of 2 eggs per day.
Means Versus Median

Don’t confuse mean difference with median, which is similar to the mean but measures a tendency based on the center of the value. To calculate the median, list your daily egg totals from highest to lowest. 3, 2, 2, 2, 1. The median point is the number from which 50 percent of your eggs came before, and 50 percent after. So in this case, that's 2. Your hen has a tendency to lay 2 eggs per day. You can also list the values from lowest to highest. In this example the mean and median are the same, but that's coincidence. They usually will be different values.
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