Mean change is a term used to describe the average change over an entire data set. The mean change is useful for comparing the results of an entire data set to see how the group performed as a whole over a period of time. For example, if you were testing a fertilizer on plants, you would want to know the mean change so you could compare the growth of the plants with the fertilizer as a group to plants that did not have the fertilizer. To calculate the mean change, you need to know the starting and ending values for each item in the data set.

Subtract the starting value from the ending value for each item in the data set. For example, if you were calculating the mean change for the change in plant height, you would subtract the starting height from the ending height for each plant.

Take the sum of the changes found in Step 1. Make sure that you decrease the total if there are negative numbers. For example, if the changes in plant height were (3, 4, 1, -1, 0, 2), the total would be nine. In this example, the -1 would represent that one plant had lost in inch of height, so the average would decrease.

Divide the total from Step 2 by the number of items in the data set. Finishing the example, you would divide 9 by 6 because the total change was 9 and the data set contains 6 items, making the mean change 1.5.

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