Calculating Percent Deviation

Percent deviation measures the degree to which individual data points in a statistic deviate from the average measurement of that statistic. To calculate percent deviation, first determine the mean of the data and the average deviation of data points from that mean.

Calculate the Mean

Calculate the average, or mean of your data points. To do this, add the values of all data points, then divide by the number of data points. Say you have four melons, with weights of 2 pounds, 5 pounds, 6 pounds and 7 pounds. Find the sum: 2 + 5 + 6 + 7 = 20, then divide by four, since there are four data points: 20 / 4 = 5. So your potatoes have a mean weight of 5 pounds.

Percent Deviation from Mean and Average

The mean and average deviation are used to find the percent deviation. Divide the average deviation by the mean, then multiply by 100. The number you get will show the average percentage that a data point differs from the mean. Your melons have a mean weight of 5 pounds, and an average deviation of 1.5 pounds, so:

percent deviation = 1.5 / 5 x 100 = 30 percent

So on average, your data points are distant from your mean by 30 percent of the mean’s value.