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##### Kevin Driscoll
(Los Angeles - United States)

I taught Computer Science and Mathematics from 2004-2007 at Prospect Hill Academy Charter School in Cambridge, MA. Since then, I completed a Master's degree in Comparative Media Studies at MIT and am now a PhD candidate in Communication at University of ...

## Variables: Dependent and Independent, and Controls

Most science investigations require a controlled experiment.  A controlled experiment is an experiment in which only one variable is tested at a time.

Investigations will often refer to both an independent and dependent variable.  Independent and dependent variables are related to one another.

The independent (or manipulated) variable is the variable that you, the experimenter, change or manipulate intentionally.

Dependent (or responding) variable is the variable that changes when the independent variable changes.  The dependent variable depends on the outcome of the independent variable.

Here are some simplified examples:

Question Independent Variable
Dependent Variable
Does water help plants grow best?
Type of liquid (water, vinegar, etc) Height of plant (in cm)
What kind of teaching helps kids learn best?
Type of instruction (reading, lecture, video, lab)
Score on test
Does the form of sugar affect how quickly is dissolves? Form of sugar (granulated, powdered, cube)
Time to dissolve (in seconds)
With each of these experiments, it is very important to control the experiment.

Question 1:  You ONLY want to test the TYPE of liquid.  Imagine you watered one plant with water every day and another plant with vinegar every other day.  If you found the plant given vinegar grew less tall, you wouldn't know if it is because of the type of liquid or the frequency of which you watered the plant.  There are TOO MANY VARIABLES.  Kids will relate to this being a "fair" comparison or not.  In this investigation, the following variables must be kept the same between the two plants:  the type of plant, size of container, amount of sunlight each day, temperature, amount of liquid, how frequently the plant is watered… etc.  These variables that are kept the same are called controlled variables.

Of course, we cannot control everything in an experiment.  We may have the same types of plant, but one seed was not as robust.  Perhaps when we planted the seed, one seed was slightly deeper in the pot than the other.  These types of control mishaps constitute "experimental error."