Counterbalancing Controlled Experiments (page 2)

By — John Wiley & Sons, Inc.
Updated on Jan 1, 2011

When to Use Counterbalanced Experiments

A simple experiment, neither uncontrolled nor counterbalanced, may be good enough for testing a hypothesis about fairly simple physical things. But for experiments in biology and in the ways people and animals behave it is difficult to hold constants steady so that only the variables change. For these experiments we need better experimental plans—controlled experiments with counterbalancing.

Will this pet mouse eat this dried pea? A simple, uncontrolled experiment will serve: The mouse eats the pea or it does not.

Can mice live healthfully on dried peas? Now things are not so simple, and you would want a controlled experiment: a control group that is fed a good normal diet and an experimental group (as evenly matched as possible with the control group) that is fed only dried peas.

Is there more fighting among mice living in crowded conditions than in conditions that are less crowded? Here it is better to use a counterbalanced plan. You would need carefully matched groups in two cages or pens, one cage or pen large enough for the number of mice and the other cage small and cramped. Keep everything else constant—food, water, litter, ages, sex, light, and temperature. After a few days of observing and recording of any aggressive behavior in each group, switch each group to the other cage and observe again for an equal amount of time.

The experiment with people that tests the effects of loss of sleep on running speed has the problem of prejudice, especially on the part of the subjects. You, the experimenter, are aware of the prejudice problems. How about the subjects of the experiment, the ones doing the sleeping and running? Do they have prejudices about the hypothesis? Surely some of them would have. And, if so, how would their prejudices affect the results? Some of them might run harder after the short sleep just to show that they were "tough;' or because they don't believe their running speed will be affected by a little loss of sleep. Others might be prejudiced in the opposite way. This prejudice among human subjects is a difficult problem. Professional scientists often deal with such prejudices by using experimental designs called "blind" and "double-blind," which we describe in the next chapter.

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