The post-promotion dip: reading what happens after a deal

The deal ended on Sunday. By Wednesday, the item that flew off the shelf all month is barely moving, and someone asks whether something went wrong. Probably not. A post promotion dip is the drop below normal sales that can follow a price deal, as shoppers who stocked up during the promotion work through what they bought.

The dip is a normal part of promotion math. Ignoring it is how promotions end up looking better than they are.

Why does a dip happen?

Mostly because of stockpiling. When a household product goes on a good deal, some shoppers buy more than they need right away. They don't need to buy again for a while, so sales in the weeks after the deal fall below where they'd otherwise be.

A second cause is pulled-forward demand. Shoppers who would have bought in week five buy in week three instead, because the price is lower. The purchase still happens. It just moves.

There's a third, smaller effect in some categories: shoppers who switched from a rival brand during the deal and switch back afterward. Their purchases during the promotion were real, but they weren't new loyal buyers. That tends to show up as a quick return to baseline rather than a dip below it.

None of these adds much lasting demand. The first two inflate sales during the promotion and deflate them after.

How do you measure the dip?

Set a baseline first, ideally from control stores that didn't run the promotion, or at least from a stable pre-period. Then compare actual sales with the baseline for the promotion weeks and for several weeks after.

Here's a hypothetical. An item's baseline is 100 units a week. During a four-week promotion, it sells 160 a week, which is 240 units above baseline. In the three weeks after, it sells 80, 85 and 95, which is 40 units below baseline in total. The gross lift was 240. The net effect, after the dip, is 200.

That difference changes the economics. If the promotion's cost was justified by 240 extra units, it looks noticeably weaker at 200. Background on sales promotion mechanics helps explain why deals behave this way.

Be consistent about the window. If you measure the dip for three weeks on one promotion and six on another, the comparisons won't be fair. Pick a standard window for each category, and extend it only when the data shows sales haven't yet recovered.

When should you expect little or no dip?

When shoppers can't or don't stockpile. Products bought for immediate consumption, such as a cold single-serve drink, are less likely to be bought ahead. Perishable items are hard to hold. Small stores with small baskets may see less stocking up than a large grocery store does.

That's an expectation, not a rule. Measure it for each item and category you promote, because the result can surprise you in either direction.

What would you do with this on Monday?

Add post-promotion weeks to every promotion review. Three to six weeks after the event is a common window, depending on how quickly the product gets used up. If your current reviews stop on the last day of the deal, they're overstating results.

It's also worth checking the dip at the brand level. If shoppers stocked up on your promoted item, they may skip your other items for a few weeks as well.

Then compare promotions by net effect, not gross lift. You may find that a modest deal with little dip beats a deep deal with a big one. That's a useful finding to bring to your distributor or sales team before the next promotion calendar is set. For broader category trends in the months around your promotions, the NRS Insights monthly reports are a helpful reference.