I am learning to work in statistics and I am currently working with a test data set that has baskets from around 200 persons that buy items from a imaginary store. They buy for example three units of milk the one day and the other day two units. I want to find out if the users in general tend to buy the same amount of items each time they are making a purchase over a period of time. Say if the customer buys milk do they always buy 3 units of milk or do they buy randomly distributed number of units of milk like (1 unit, 2 units, 1 unit, 5 units, ...). I will need to do that with every customer and every product group and conclude if in general customers buy the same amount of products for each purchase.
My idea was to calculate the average difference between all purchases like this:
(1-2) + (2-1) + (1-5)/4 (using the example above)
for each user and each product but that seems to me like it does not tells me anything important. Is there maybe a statistical and more correct approach to answer this question?
Thanks a lot in advance to the stack exchange community!