The researcher compares the results of an experimental sample against those from a control sample that is treated the same, except for the factor under investigation. Experimental research questions must include an independent variable, dependent variable, and subject (see sections 5.5-5.6).
The independent variable is the variable that the researcher selects or manipulates to determine if it produces changes in the dependent variable. The researcher looks for evidence that changes in the independent variable are correlated with changes in the dependent variable. Establishing a relationship between the independent and dependent variable may suggest or confirm cause and effect. The independent variable is independent of the dependent variable. It may be changed or controlled by the experimenter, but it is not affected by changes in the dependent variable. By contrast, the dependent variable is (potentially) dependent upon the independent variable. If there is a relationship between the independent and dependent variables, then changes in the independent variable will result in changes in the dependent variable. The researcher should hold all other factors constant to make certain that changes in the independent variable are responsible for changes in the dependent variable.
Researchers must specify the subject of the study to clarify the extent to which one can generalize the findings. For example, a researcher may demonstrate that the optimal elevation for the giant redwood (Sequoiadendron giganteum), the largest living species, is 1400-2000 meters in California’s Sierra Nevada Mountains. One cannot, however, say this is the optimal elevation for the growth of the closely related coastal redwood (Sequoia sempervirens), the tallest living species that grows near sea level in California’s Coast Range. The researcher must state the subject and conditions to which his or her findings apply. The conditions are the parameters under which the study is conducted, and may include any of a variety of key factors such as temperature, light intensity, and pH. There are many ways to write an experimental question, but the following formulas work well because they include independent variable, dependent variable, subject, and conditions.
"What is the effect of (independent variable) on (dependent variable) in (subject) at (conditions).
“What is the relationship of (independent variable) to (dependent variable) in (subject) at (conditions).