This book is based on three key propositions. First, the effectiveness of any educational, therapeutic, or counseling intervention depends at least in part on the quality of the assessment on which it is based. For example, a decision to provide a youth with a program of behavior modification treatment for depression would be appropriate to the extent that it reflects the actual emotional state of the youth and other characteristics that would affect his or her reactions to this treatment. Similarly, the ultimate success of a decision to counsel a youth to follow an academic stream in secondary school will be limited by the accuracy of the initial assessment of the student's aptitudes and interests.
Second, standardized psychological assessments and structured diagnostic procedures are more likely to yield valid information about an individual than the informal and unsystematic assessments so often employed in clinical, counseling, and educational settings. There is certainly a role for experienced clinical judgments in the treatment of clients, but, as I try to show in later sections of the book, standardized psychological assessments provide a firmer and more defensible foundation for diagnostic and decision-making activities.
Third, assessment tools and procedures should be appropriate for the individual being assessed. I have chosen in this book to focus on the assessment of adolescents because I believe that this group often exhibits characteristics and circumstances that set them apart from children and adults. Further, these unique features mean that, in many cases, assessments and interventions appropriate for younger and older age groups may not be indicated. This point is reinforced in the discussion of theory and research on adolescence that follows.