The experimental approach that we have illustrated above involves establishing two situations that differ with respect to only one factor (variable). This factor will determine if the prediction of the hypothesis is correct. The situation with the varying factor is referred to as the experimental group. The unmanipulated situation from which the experimental group varies is called the control group. Any difference between the experimental group and the control group should be due to the single varying factor, the factor that will test the research hypothesis.

The factor that is manipulated or controlled by the experimenter is the independent variable. This is the variable being investigated; in the example above, the independent variable is whether or not the vein was cut. The dependent variable is the factor being measured, which is dependent on the independent variable. In our leaf example, the dependent variable could be the turgor or health of the leaf. That is, the health of the leaf is dependent on whether or not the vein has been cut.

In many areas of science, it is impossible to use the experimental approach to hypothesis testing, usually because it is not feasible to create an experimental group (that is, to impose a treatment). This is clearly the case with sciences involving interpretations of past events such as geology. In the biological sciences, hypotheses in the disciplines of evolution, ecology, and animal behavior are frequently not amenable to experimental testing and must be approached using a non-experimental or observational approach. For example, it has been observed in many species of hawks that the female is significantly larger than the male. It has been hypothesized that this size difference allows the male and female hawks to capture prey of different sizes, with the female taking the larger prey, the male, the smaller prey. One possible advantage of this would be less direct competition for food between a pair of hawks as they try to raise their young. But how could this hypothesis be tested experimentally? Still, the hypothesis makes certain predictions which, if found to be correct, test the probable validity of the hypothesis. What are some of the predictions testing the above hypothesis? How would you gather the necessary data?

The non-experimental, or observational approach to hypothesis testing, like the experimental approach, involves a search to determine if the predictions made by the research hypothesis are correct. The search usually involves going out into the real world and making additional observations. If male and female hawks take different-sized prey, one should be able to observe it, either by directly determining what the birds are eating or by examining the non-digestible remains (castings) of what they have eaten. The null and alternative hypothesis, in this case, would be stated:

Ho: There will be no observable size difference in the prey captured by male and female hawks.

Ha: There will be an observable size difference in the prey captured by male and female hawks.

Both approaches are concerned with determining if the predictions of a hypothesis are correct. The experimental approach involves manipulating some characteristics of the phenomenon being studied and comparing it with the unmanipulated state. The nonexperimental approach involves making additional pertinent observations suggested by the research hypothesis. Both approaches are valid scientifically. One of the major biological theories of all time, natural selection (Darwin, circa 1860), was formulated with a non-experimental approach.

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