## Formula for what parts to keep in stock

Hi everyone,
I've got what I think is a maths problem, however Im not sure if it belongs in this category.

Anyhoo..

Here's the background to my situation:
I've recently inherited the role of manager of a department that provides service on photocopiers. So I have a team of engineers who attend our customers equipment when something goes wrong.
Often they have to fit part(s) which they have with them to get the equipment running again.

The problem:
Im looking to put in place a system for ensuring that only parts that get used often, are ordered for stock, and thought something like a "Popularity metric" might be the way ahead.

The factors I want to include are:

1. How recently the parts were fitted - Because I dont want to stock parts for old equipment
2. The quantity of an individual part fitted - Obviously!
3. Over how many calls - Im including this because in some cases, 22 of a particular part all get replaced at once, but this may only happen once or twice in a year

Here's what Ive come up with so far:
Per part:
Starting with A numerical representation of the date (D)- Which I think is the number of seconds since 1st Jan 1980
The qty fitted since (01 Jan 2010) (Q)
Over how many calls has Q been fitted over (C)

Im defining a "popular part" (e.g. one we want to keep in stock) as being fitted on over 4 separate calls (T)

So here's the forumla Ive come up with:
Where P = "Popularity"
P= D + ((C-T)*Q)

The idea for the above being that unless the C is over the call count threshold (T) then it will produce a negative number, and subtract from the inital date value (D) thus making parts that are fitted 22 at a time on one call, less popular than a single part fitted over 20 calls (For instance)

My formula is there or there abouts, but the date is affecting things too much.
At the number 1 spot is something we've fitted 53 times over 50 calls, but at number 2 is a part we've fitted 6 times over 6 calls but for the whole year! The date it was fitted obviously having an effect there.

Im really asking to see if there is a better / more standard way of achieving this, or is my somewhat simple effort just in need of a bit of tweaking?

So thats pretty much it. Im afraid my knowledge of maths doesnt extend much past secondary school, so be gentle, its my first time! ;D