The present paper is one of a series of reports by the authors on empirical research which has been conducted by the authors since 1967 into the problems of introducing new products. A survey of contemporary practice has highlighted the areas which are of particular concern and has led to the development of new techniques and methods of improving managerial performance in implementing existing techniques. Specific attention is paid in this instance to the key problems of grossing up test data to national equivalent levels. After examining the role of grossing up within the wider framework of test marketing activity, we shall present a number of alternative methods, some of which are currently in use, and assess the validity of these measures. It is the purpose of the present paper to examine the soundness of this procedure and to suggest ways in which we may approach more closely this fundamental goal of understanding the reaction of the market to the new offering, in order that a decision to extend distribution can be made with confidence.