Why is a control especially important in biological investigations?

Answer 1

There are three types of variables in a scientific experiment:

Independent: Changes which you, the experimenter, control.

Dependent: Changes which occur based on the changes you make (Independent)

Control: Anything else which might change or influence the dependent variables outside of the independent changes made by the experimenter.

Control variables must be monitored and controlled during an experiment to make sure that they are kept equal, otherwise they could make your results false or unreliable.

In the Cool Science Projects link, they discuss a plant growing project. You wish to determine the growth difference between plants which have a full eight hours of sunlight versus plants which receive four hours of sunlight. The amount of time the plant is in the full sunlight is the independent variable and one of the dependent variables is the growth rate of each plant. There may be other dependent variables, such as the overall height of each plant.

A good example of a control variable might be the amount or type of water used to hydrate the plants during the experiment. If you were to give one plant more water than another, or different qualities of water, you would be further influencing the experiment beyond the initial independent variable. You could not then say that it was strictly the duration of time in the sun which caused the growth or height differential, as it may have been other variables which caused these changes.

It's important to be aware of exactly what changed within an experiment, and what did not, in order to keep your findings valid.