There are 2 main fields. In the upper field you can generate the average value by having the sum of all values and divide it to the number of the values. For example take a 38 months visits statistics in Google Analytics. It shows you the sum of all visits for this period. Enter this value in the imput field under "Sum of all values". Then because it is for 38 months period, enter this value in the imput field under "Number of Values". It will automaticaly generate an average value in the output field. It shows 233, which means you have 233 visitors on average per month. In the second lower field, you can find average value by entering series of data and the sum, the period and the average is calculated automatically. It may be used for finding average school mark. Just enter the school marks for all school subjects.
