I have a very beginner question. I am attempting to forecast total 2014 unit sales of a large number of products.
The data I have has 10 points for each individual product, which are the total unit sales for each of the previous 10 years. I also have the total yearly store revenue for the last ten years, which I can use to establish an overall average growth.
I don't want to use a simple arithmetic mean, because I want to account for regression on products that performed exceptionally well or badly in 2013.
I'm assuming the variation is random, and am not trying to account for promotions or seasonality.
I'm also assuming that the base growth rate for each product is the average growth rate of total store revenue.
I just want a reasonably accurate prediction of total unit sales for each product to set guidelines for inventory purchasing.
I am working in Open Office Calc, and I also have R, but am a total beginner in using it. What would be the best way to approach this problem, and how do I implement it in these programs?