Single-Factor Designs: Simple Experimental Designs That Incorporate the Rules for Research
In chapter 3's discussion of rules for designing research it was mentioned that avoiding confounding factors is a difficult task. The experimenter is greatly aided in this task by the existence of standard experimental designs, the use of which lessens the probability of confounding factors. This chapter discusses the most common of these designs. Their use minimizes confounding factors in both manipulated and subject variable experiments, although the probability of completely eliminating confounding factors is unlikely when using subject variables.
It helps to appreciate the advantages of the standard experimental designs if they are compared with an improper design that should not be used because of its high susceptibility to confounding factors. Unfortunately, as explained later, this improper design is informally used sometimes by people drawing conclusions from personal experiences. When people use it, they do not call it an experimental design, and it is certainly not being recommended here as an experimental design. Rather, it is a way in which people sometimes reason. But it is useful to see precisely how this informal way of thinking and drawing conclusions is both similar to and different from a standard experimental design. This will make it easier to understand in what ways standard experimental designs offer better ways of determining just what is related to what in our world.
Problems encountered in designing and implementing experi-