I'm trying to calculate on average, how long it takes to make each item.

Here is the data table:

Date        Item    Quantity   Operator
01/01/2014  Item1   3          John
01/01/2014  Item2   5          John
02/01/2014  Item1   7          Bob
02/01/2014  Item2   4          John
03/01/2014  Item1   2          Bob
07/01/2014  Item2   3          John

On 01/01/2014 John made 3 of Item 1 and 5 of Item 2. If we only had the first 2 rows we can guess that it takes 0.375 days to make Item 1 and 0.625 days to make Item 2.

I want to be able to calculate this on average using all the data and taking in to account the operators obviously working on different items.

Thank you