What are variables?

Guide to identifying dependent, independent and controlled variables.

How do I define variables? 

If you increase the temperature of an oven, cake will bake faster. In other words the TIME it takes for the cake to bake DEPENDS on the TEMPERATURE of the oven.

If your project title is "How does the oven temperature affect the baking time of a cake?", one thing that you need to do is identifying the variables.

In this example BAKING TIME and OVEN TEMPERATURE are the two variables that we measure, record and use them to draw a conclusion. 

Temperature of oven is what you set. For example you may set temperature to 350F and bake a cake. Temperature is what you manipulate and is not depended on any other factor. That is why you call temperature an independent variable. It is also called manipulated variable. We call it manipulated variable because you manipulate or change it in order to see how does it affect the baking time.

On the other hand, the BAKING TIME depends on the temperature. That is why you call it DEPENDENT variable. Changes in the BAKING TIME is in response to the TEMPERATURE. So you can also call it RESPONDING variable.

While you are doing your experiments, some other changes may happen in the environment that make your results unreliable. For example you may test baking a cake in the morning at 350F and another cake in the afternoon at 400F. You may bake one of them in a small oven and the other one in a big oven. You may bake one cake in the lower shelf of the oven and the other one in the upper shelf of the oven.

Such variations will make your results unreliable. For example if your conclusion shows that "cake will bake faster at higher temperatures", someone may challenge your results and argue that "One of the cakes baked faster because it was in a smaller oven, or because it was at the lower shelf or because it was baked in the afternoon". How do you know that the temperature was the only factor affecting the baking time?

To avoid such arguments, you also define some controlled variables and some constants. For example you may use two identical ovens and cook both cakes at the same time and use the upper shelves of the ovens. 

Independent variable (also known as manipulated variable) is a condition or factor that we change or choose in order to see how it affects other variables.

Dependent variables (also known as responding variables) are conditions that are affected by any change in the independent variable.

Controlled variables are variables such as room temperature, light and other environmental factors that you cannot keep constant; however, you can make sure that all experimental runs are performed under the same conditions.

Constants are values such as the size of oven, the size of pan, the ingredients of the cake, the size of the cooking pan, the amount of material.

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