Experimental Design

From Scientificmetho

"To minimize individual differences by reducing the effects of extraneous variables, in order to and draw predictions of behavior from a experimental stimulus."

One Factor - Two Level Designs

Ok, the simplest experimental design we have is a design with a single independent variable and two levels of that variable. The two levels in the simplest design would be one group getting the independent variable (the experimental group) and one group getting no treatment (the control group) but being treated in an otherwise similar fashion (placebo).

The independent variable can be tested between groups as in the above example, or within groups. If it is tested between groups, it could either be a manipulated (again, as above) or a subject variable. If the independent variable is manipulated, the design will either be called an independent groups design if random assignment is used to create equivalent groups, or a matched group design if matching is needed. If a subject variable is being investigated, the groups are by definition a nonequivalent group. Terman’s Termites, members of a longitudal study designed to see if intellectually gifted people (such as me) lived better lives than people of average intelligence (such as you), were subjects involved in an nonequivalent study. Matching was used to reduce the nonequivalence as much as possible, such as picking people of the same age groups, same percentage of gender, same racial groups.

The repeated measures design is a within group design, used when the independent variable is tested within subjects, or when every subject in the study experiences both levels of the independent variable. A typical example would be food taste testers. This design has the benefits of requiring less subjects than between group designs, since every participant will experience all levels of the independent variable. This design also reduces the errors that occur when comparisons are made between groups, since the exact same subjects are being compared to themselves! However, this design suffers from experimental confounds that the between groups designs do not - dealing with the confounds that prior experience with the independent variables cause, such as proactive interference - where the learning of new material is hindered by similar old material previously learned. Another confound involves the effects of order of presentation.

Now, to test your knowledge of experimental design, try out the decision tree below. Just hold your mouse over the title and try it out.