Internal and External Validity in Clinical Research

dvances in clinical psychology critically depend on methods researchers use for investigating the causal relationships among variables. Research questions commonly include issues about the putative mechanisms associated with various forms of psychopathology (e.g., do dysfunctional beliefs cause depression?) and questions about the effects of treatments (e.g., does cognitive therapy cause a greater reduction in eating disorder symptoms than does placebo?). How do we go about drawing objective conclusions about causality? This is a question about whether the manipulation of one variable (i.e., the independent variable) has effects on another variable (i.e., the dependent variable). The answer to this question is more complex than it might seem. Consider the following common scenarios.