Some Basic Design Concepts Experimental Design

Sir Ronald Fisher, the statistician, eugenicist, evolutionary biologist, geneticist, and father of modern experimental design, observed that experiments are ‘only experience carefully planned in advance, and designed to form a secure basis of new knowledge’ (Fisher, 1935: 8). Experiments are characterized by the: (1) manipulation of one or more independent variables; (2) use of controls such as randomly assigning participants or experimental units to one or more independent variables; and (3) careful observation or measurement of one or more dependent variables. The first and second characteristics—manipulation of an independent variable and the use of controls such as randomization—distinguish experiments from other research strategies. The emphasis on experimentation in the sixteenth and seventeenth centuries as a way of establishing causal relationships marked the emergence of modern science from its roots in natural philosophy (Hacking, 1983). According to nineteenth-century philosophers, a causal relationship exists: (1) if the cause precedes the effect; (2) whenever the cause is present, the effect occurs; and (3) the cause must be present for the effect to occur. Carefully designed and executed experiments continue to be one of science’s most powerful methods for establishing causal relationships.