Responsiveness is quickly becoming a critical criterion for the selection of outcomes measures in studies of treatment effectiveness, economic appraisals, and other program evaluations. Statistical characteristics, specifically "large effect sizes," are often felt to indicate the relative worth of one instrument over another. However, debates about their meaning led the present authors to propose a taxonomy for responsiveness based on the context of the study concerned. The three axes underlying the classification system relate to: who is this being analyzed for (individuals or groups); which scores are being contrasted (over time/at one point in time); and the type of change being quantified (for example, observed change or important change). It is concluded that responsiveness should be considered a highly contextualized attribute of an instrument, rather than a static property and should be described only in that way. A questionnaire could thus be described as being "responsive to" a given category in the new taxonomy.