I'm not much of an analyst here, but a couple of thoughts spring to mind just from a logic standpoint. You want to calculate the volume you should have in the shop based on data from a previous year, yes? First, you should probably have a full year of data to start with so that you have some sort of base to work from. Perhaps even more so you can calculate variances over a period of years.
Second, are you anticipating that volume should remain static, or potentially increase or even decrease over the forecast time frame? If you think that you should increase volume 2% over the forecast time frame, then you would calculate the existing volume for the previous month/year times 2% and produce your estimates accordingly. Now, the question then comes what happens if your estimated forecasts go wildly awry (e.g. - storing generators that would have a low turnover rate unless a sudden demand occurs due to hurricane-related power outages which would then skew your numbers for that given month)? Do you then change the subsequent month's forecast to account for the previous month's variation in addition to your original forecast or what?
Ok, think I'm getting way ahead of myself here and not sure if this is what you are looking for. For that matter, are you seeking the coding for a logical process that you already know or the logic when you can then code? I'll wait to hear back before I jump into any more high water. ;-)
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If you don't have a sense of humor, you probably don't have any sense at all.
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