Students often find it difficult to understand what an independent and dependent variables are in an experiment. They also struggle with what a control group, experimental group and controlled variables are. Once you understand what each is you’ll realize there is nothing to be confused about.
The terms “independent” and “dependent” already give you a clue as to what they are. The dependent variable depends on the independent variable. Think of the dependent variable as the “result” of something. The independent variable is the “cause” then.
Let’s use an example. Let's say you have a fish tank and your fish are dying every day. You realize that you recently started feeding your fish Walmart brand fish food and that’s what is causing them to die.
Here, the result is the fish dying. In other words, your dependent variable is the fish dying. The cause of their dying, Walmart fish food, is the independent variable.
The independent variable does not depend on anything.
Dependent variable-” the result”: fish dying
Independent variable-” the cause”: Walmart fish food
In science, an independent variable is a variable that is manipulated in an experiment in order to observe its effect on a dependent variable.
Now that we understand what dependent and independent variables are lets move to the next three concepts.
A control group is a group in an experiment that serves as a benchmark to compare the results of the experimental group. The control group is usually not exposed to the treatment or manipulation being tested.
An experimental group is the group in an experiment that is exposed to the treatment or manipulation being tested. In other words the experimental group receives the independent variable. The results of this group are then compared to the results of the control group.
Controlled variables are any factors that are kept constant during an experiment. This is done to ensure that any differences in the results between the experimental and control groups can be attributed to the independent variable being tested.
Lets revisit our fish situation. We will now test if it is truly the Walmart fish food that is killing the fish. We will divide our fish into two groups: the control group and the experimental group. The experimental group will receive Walmart fish food (the independent variable), the control group will receive the fish food you had been using before you switched to the Walmart brand. Your controlled variables could be a number of things: keeping the same temperature in both tanks, keeping the light levels, oxygen levels the same in each tank.
Having controlled variables rules out any potential chances of error. When you keep all conditions (other than the manipulation-fish food) constant you can conclude that your results are due to the fish food and not anything else.
You run your experiment for a week and note the results. Results can be measurable or quantitative i.e. how many fish died. Results can also be qualitative or observable i.e. the fish were more active. If at the end of the week more fish in the experiment group die you can conclude that it is indeed your suspected independent variable (the Walmart fish food) that is causing the fish to die.