The paper deals with two broad areas of bias in the preconceptions of the respondent and the preconceptions of the researcher. In relation to the former the specific areas of bias dealt with are: A. the motivation of the respondent; B. the presentation of product samples; C. the words used to name product qualities. In relation to the latter preconceptions the specific areas of bias dealt with are: A. relationships selected for interpretation by the researcher; B. the questionnaire design. The basic principles in each of these specific areas are discussed and an attempt is made to illustrate how the principles may be translated into operational terms to eliminate these sources of bias, for example: A. Three call method of comparative testing; i. e. placing only one product at a time for a consumer's evaluation; B. Prescriptive scales; i. e. the respondent indicates how he would like the product to be rather than describing the way he thinks it is; C. Hierarchical statistical analyses; i. e. analyses which are structured in a manner corresponding to the hypotheses and questionnaire.