The correlation that may exist between advertising pressure and effectiveness can be expressed as a response function for marketing purposes, perhaps the most interesting relationship is that which may exist between media exposure and sales, and it is in this context that the term "response function" is used here. The allocation of expenditure between different media and the problem of intermedia comparisons can, by relating the different response functions, move the solution from an area of subjective estimation to one of scientific method. An equally attractive prospect for planning with individual media makes it worthwhile investigating the utility of product/media exposure patterns. Such relationships may enable the media planner to construct 'optimum' schedules against 'target' audiences, defined in terms of probability of purchase. Both areas of investigation are exciting, both raise the possibility of introducing scientific measurement to replace more subjective methods of analysis. The problem of measurement in both cases depends on data collected through product/media surveys from the same individuals about their exposure to various media, and their purchasing behaviour over a range of product fields. Before these exercises could be attempted however, there were two problems to be solved. These may be described in terms of the validity and stability of the information to be used.