Define the Variables

There should be three categories of variables in every experiment: dependent, independent, and controlled.

Dependent -- is what will be measured; it's what the investigator thinks will be affected during the experiment.

For example, the investigator may want to study coffee bean growth. Possible dependent variables include: number of beans, weight of the plant, leaf surface area, time to maturation, height of stem.

Independent -- is what is varied during the experiment; it is what the investigator thinks will affect the dependent variable.

In our coffee bean example, possible independent variables include: amount of fertilizer, type of fertilizer, temperature, amount of H2O, day length, all of these may affect the number of beans, weight of the plant, leaf area, etc.

Key : Since you need to know which factor is affecting the dependent variable(s), there may be only one independent variable. The investigator must choose the one that he/she thinks is most important. But the scientist can measure as many dependent variables as he/she thinks are important indicators of coffee bean growth.

Controlled -- the variables held constant. Since the investigator wants to study the effect of one particular independent variable, the possibility that other factors are affecting the outcome must be eliminated.

For example, the above scientist must ascertain that no differences in the type of fertilizer used exists, or amount of H2O, variations of temperature, or day length exist.

Main Page | Introduction and Objectives | Scientific Investigation | Experimental Procedures | Writing Procedures | Mendelian InheritanceMonohybrid and Dihybrid Exercises | Reference | Miscellaneous | Overview | The Question | The Hypothesis | The Experiment | The Scientific Paper | Variables | Procedures | Predictions | Protocol | Data Evaluations | Conclusions |