A well-designed experiment copes with the potential effects of extraneous variables by using random assignment to experimental conditions and sometimes also by incorporating direct control and/or blocking into the design of the experiment. Each of these strategies—random assignment, direct control, and blocking—is described as follows; A researcher can directly control some extraneous variables. In the calculus test example, the textbook used is an extraneous variable because part of the differences in test results might be attributed to this variable. We could control this variable directly, by requiring that all sections use the same textbook. Then any observed differences between temperature groups could not be explained by the use of different textbooks. The extraneous variable time of day might also be directly controlled in this way by having all sections meet at the same time. The goal is to design an experiment that will allow us to determine the effects of the explanatory variables on the chosen response variable. To do this, we must take into consideration any extraneous variables that, although not of interest in the current study, might also affect the response variable.
The effects of some extraneous variables can be filtered out by a process known as blocking. Extraneous variables that are addressed through blocking are called blocking variables. Blocking creates groups (called blocks) that are similar with respect to blocking variables; then all treatments are tried in each block. In our example, we might use instructor as a blocking variable. If five instructors are each teaching two sections of calculus, we would make sure that for each instructor, one section was part of the 65° group and the other section was part of the 75° group. With this design, if we see a difference in exam scores for the two temperature groups, the extraneous variable instructor can be ruled out as a possible explanation, because all five instructors' students were...
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