Best research evidence refers to scientific results related to intervention strategies, assessment, clinical problems, and patient populations in laboratory and field settings as well as to clinically relevant results of basic research in psychology and related fields. A sizeable body of evidence drawn from a variety of research designs and methodologies attests to the effectiveness of psychological practices. Generally, evidence derived from clinically relevant research on psychological practices should be based on systematic reviews, reasonable effect sizes, statistical and clinical significance, and a body of supporting evidence. The validity of conclusions from research on interventions is based on a general progression from clinical observation through systematic reviews of randomized clinical trials, while also recognizing gaps and limitations in the existing literature and its applicability to the specific case at hand (APA, 2002). Health policy and practice are also informed by research using a variety of methods in such areas as public health, epidemiology, human development, social relations, and neuroscience.

Researchers and practitioners should join together to ensure that the research available on psychological practice is both clinically relevant and internally valid. It is important not to assume that interventions that have not yet been studied in controlled trials are ineffective. However, widely used psychological practices as well as innovations developed in the field or laboratory should be rigorously evaluated and barriers to conducting this research should be identified and addressed.

Psychologists' clinical expertise encompasses a number of competencies that promote positive therapeutic outcomes. These competencies include:

a) Conducting assessments and developing diagnostic judgments, systematic case formulations, and treatment plans;

b) Making clinical decisions, implementing treatments, and monitoring patient progress;

c) Possessing and using interpersonal expertise, including the formation of therapeutic alliances;

d) Continuing to self-reflect and acquire professional skills;

e) Evaluating and using research evidence in both basic and applied psychological science;

f) Understanding the influence of individual, cultural, and contextual differences on treatment;

g) Seeking available resources (e.g., consultation, adjunctive or alternative services) as needed; and

h) Having a cogent rationale for clinical strategies. Expertise develops from clinical and scientific training, theoretical understanding, experience, self-reflection, knowledge of current research, and continuing education and training.

Clinical expertise is used to integrate the best research evidence with clinical data (e.g., information about the patient obtained over the course of treatment) in the context of the patient's characteristics and preferences to deliver services that have a high probability of achieving the goals of treatment. Integral to clinical expertise is an awareness of the limits of one's knowledge and skills and attention to the heuristics and biases—both cognitive and affective—that can affect clinical judgment. Moreover, psychologists understand how their own characteristics, values, and context interact with those of the patient.