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.
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.
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