by Serguey Shinder
In January 2025 a plasterer drove forty minutes to our branch in south Bristol to collect thirty bags of finishing plaster he had reserved on our website that morning. The website had said forty two in stock. The branch had eleven, and the counter staff had known since breakfast that it would have about that many by lunchtime.
The cause was a rule written into our stock system long before I arrived, and it looked entirely sensible. Stock could never go below zero. If a sale would take a product negative, the system recorded the sale and set the quantity to zero. Somebody had once been tired of seeing minus figures on reports, and nobody can hold minus four bags of plaster, so the rule had survived every review since.
On a busy branch, the order in which things are recorded is not the order in which they happen. That morning a lorry arrived at half past six with fifty bags. Before anybody had booked the delivery in, which needs the paperwork and a free pair of hands, the counter sold thirty one bags straight off the pallet to the early trade. The system saw stock of zero, sold thirty one, and kept it at zero. At ten the delivery was booked in, the system added fifty, and announced fifty. Eight more sales took it to forty two. The shelf held eleven.
The negative number had been the truth. Minus thirty one meant that we had sold goods not yet booked in, and that a receipt was on its way. Clamping it to zero did not make the stock more accurate. It deleted the one piece of information that would have put the figure right once the paperwork caught up, and replaced it with a number that looked tidier and was wrong by exactly the amount we had sold.
Across all our branches, about four per cent of products had been through this at least once in the previous quarter. Mostly fast movers on busy counters, which were exactly the products customers reserved online.
We took the rule out. Stock can be negative now, and a negative figure is treated as a question rather than an error. It appears on a list for the branch, which usually means a delivery is waiting to be booked in, and it clears itself when it is. The website shows nothing as available while a product is below zero. And the reports that used to show tidy zeros now show the minus figures, which the branch managers asked to keep once they understood what they meant.
The plasterer had his thirty bags delivered to his job that afternoon from our other Bristol branch, at our cost.
A number that looks impossible is often telling you the order in which things happened. Forcing it to look possible throws that away.
– Serguey Asael Shinder
Leave a Reply