Buying forecast

Question

I have a set of items. In 2013, I bought x of a certain item, in 2014 I bought y, and in 2015 I bought z.

In many cases, I only bought the item in one year, like:

2013    2014    2015
15      0       0

Would the best forecast of what I will buy in 2016 be

(2013 + 2014 + 2015) / 3

Or just what I bought in 2015?

Answer 1

One could also say that the "best forecast" is $0$. Consider the case where you bought $10000$ items in year 2006, but then $0$ items during the following $9$ years. Intuitively, one would expect you to buy $0$ items again in year $2016$, not depending on what you bought in $2006$.