In this article we show how the effects of many marketing actions can now be evaluated in some detail. The basic question considered is not whether there was any positive effect at all (eg on total sales) but what kind of effect it was (eg extra penetration or heavier purchasing from existing customers). To make such evaluations, we need to compare what actually happened with what would have happened without the marketing action. Research in the last decade has facilitated such comparisons without having to run controlled experiments. Instead of having to measure directly what would have happened without the marketing action, it is now possible to predict such norms successfully. The case-history described in this article is one where a controlled experiment could in fact have been mounted, if it based on a study for the J. Walter Thompson Company had been planned in time. The special lesson therefore is to show how an evaluation could actually be carried out after the event, and how ibis was in any case much cheaper. The approach adopted has already been applied in evaluating many marketing situations, such as price-changes, new brands, relaunches, seasonal trends, life-cycle assessments, private label brands, and various kinds of consumer and retail promotions. As a specific example we describe here a case-history which involved a consumer promotion or deal'.