Sir, I would like to applaud The Grocer 33's mystery shopping service's new formula for availability (Leader, The Grocer, 30 October).

Promotions are for people. The Grocer's improved way of looking at shelf availability needs to bear in mind the empty holes recorded are at peak periods, where three or more times the average number of shoppers are in stores. Therefore the 5% additional impact on shelf is that much greater on the shopper.

It is a shame that retailers and sales directors are in denial over these figures. The fastMAP 1000 plus research sample we use is very robust, and the consistent picture it brings us is of shoppers using coping strategies to get their hands on discounted stock. When they fail, they will blame the brands as well as the retailer. This means the investment companies make in building brands with one hand is thrown away on a daily basis with the other. Increasing shopper cynicism is not a good foundation for building future trust.

Colin Harper Head of insight, Institute of Promotional Marketing