An Automated Replenishment System

May 8, 2009

There’s an assumption, these days, that systems whose basic design principles and strategies have been around for twenty years are, by this time, working pretty well. If you think that, consider the following.

A person I know well was asked to evaluate the effectiveness of some automated replenishment software at a retailer of his acquaintance. The automated replenishment system had been built to optimize inventory levels, so it automatically asked for replenishment when inventory was low.

What he found was that the store managers were systematically ignoring the replenishment requests and overriding them. Hah. We all know about this, right? Stupid, willfull store managers ignoring the mathematically sophisticated system that was calculating real supply and demand. The real trouble with business is people, right?

Not at all. It turns out that the best way to optimize inventory levels is to delay orders. (First day, Supply Chain 101). The peak days for the stores were Thursday, Friday, and Saturday. Since the stores got daily deliveries, taht meant that the peak replenishment days were…Thursday, Friday, and Saturday. Unfortunately, the store managers didn’t want deliveries on those days. They didn’t have the staff for it. They wanted deliveries on slow days, when the staff would be available for it. So every week, they were going into the system and moving the replenishment requests forward to Monday and Tuesday. Every single week. For thousands of SKUs.

Now there’s a good use for managers’ time.

My friend is a skeptical sort, though very helpful. So before throwing up his hands, he tried to decide whether they were doing the right thing. He wrote an optimization algorithm that optimized deliveries based on inventory levels AND labor costs. Guess what. The store managers were doing exactly the right thing. His algorithm slightly outperformed the managers, probably because they had to do all that changing by hand. But the outcomes (the actual outcomes by the store managers and his theoretically optimal outcomes calculated by his algorithm) were very close.

And yes, he did leave the algorithm behind him, so now the managers don’t have to do it by hand any more.

So here’s what we know. Basically, the people who wrote the automated replenishment system had done a bad job. They had written an “advanced,” “automated,” replenishment system that optimized decision-making, but they had optimized on the wrong thing. So instead of providing a system that did optimal, automated ordering, they created a system that generated thousands of orders, each of which had to be corrected by the highest paid person at the store.

And until my friend came along, nobody seemed to know about it.

Now, I’m not saying who is responsible for this, whether it is a commercial replenishment system or a custom-built replenishment system. But what I can say is that all the commercial replenishment systems that I know about do it in exactly the same way; they optimize around inventory levels.

Why do they do that? Well, for one thing, the system is self-contained. They have the inventory information available in the same system, so adding in an optimization doesn’t require any integration.

For another, the people who are building systems like this do not think like retailers (even though their customers are retailers). If they did, they would realize that the orders need to be staggered.

And for another, neither they nor their bosses nor the people who bought the system never, ever followed up on what they did. If they had followed up, they would realize that the system they built was not doing the job that it should.

What it should have done is generate the right orders automatically. But all it was actually doing was generating template orders all of which had to be inspected and corrected by hand. It wasn’t a complete wash. It did generate orders with roughly the right amounts (over a week). And, because of the way the system worked, some of the corrections were relatively rapid. (Sometimes, if you moved two orders forward, subsequent orders would automatically self-correct, because the system would reduce subsequent orders by the amount that had already been ordered.)

So is this some isolated instance? I don’t think so. As you probably know, the basic operation and design of these systems has been around for twenty years. All the commercial systems, so far as I know, optimize around inventory, without taking labor availability into account. If this particular system works very poorly because of a design error, there’s every reason to believe that others don’t work either.

If that’s the case, it’s a little scary. There have been a lot of automated replenishment systems put in over the past twenty years, and almost all of them have been justified by the claim that they kept inventory levels down to optimum levels. If that claim is wrong (because keeping inventory low necessarily entails excess labor costs), then there are a lot of systems out there that were sold and installed under false pretences and are now being used incorrectly.

The ironic thing about this is, of course, is that only a very little attention to how these systems are being used would not only show you the problem, but also show you the solution. To correct the system, you don’t need to integrate with a labor availability system (that would be hard and isn’t what my friend did). All you’d need to do is write a small program that corrected the timing, using the corrections that the humans were doing already as a guide.

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2 Responses to “An Automated Replenishment System”


  1. It seems like the basic problem is the inventory optimization software had lead times configured incorrectly. The lead time should include the time it takes to prepare a purchase order and the time it takes to process inward goods – not just the time it takes to transport the goods… so a simple fix would be to bump up the lead time by 3 days right (the time to process inward goods)?

  2. toppundit Says:

    You misunderstand. The system I’m talking about was configured perfectly; it did exactly what it was designed to do, optimize on inventory costs. If you bump up the lead times (that is, if you have the goods arrive three days before they’re needed), then you’re de-optimizing, because now you have to pay carrying costs on the inventory for three days. No self-respecting programmer or MRP expert who is paid to do what MRP systems do (optimize on inventory costs) would ever do that, because that would be wrong morally, and besides it would lessen the ROI of the software they’re building.

    So weren’t the managers de-opimizing? Yes indeeedy-pooh, they were. They were de-optimizing relative to inventory carrying costs. But they were optimizing relative to other factors: labor costs and revenue, since having stuff on the shelves increases revenue. And, as my friend showed, their optimization was correct. The programmers was not.

    Your suggestion would be right if the store managers could actually set the lead time in the calculation. It would have saved them a lot of manual labor. But I seriously doubt that this was possible for them. Most of these systems attach lead time parameters to the item, not to the store-item combination. So they would have to reset the parameter (if it existed) for everybody.


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