GED Science Practice Test: Variables

Scientists design experiments very carefully in order to make sure that they specifically and meaningfully test the hypothesis.  In order to make experimental design easier to understand, the different aspects of experimental design will be tied to a specific, example hypothesis. As you read, consider the following hypothesis: if more magnesium is added to radishes, they will grow more.

One thing that scientists carefully consider in designing experiments is the selection of variables.  Variables are things that can change in an experiment.  There are three kinds of variables in experiments—the independent variable, the dependent variable, and the controlled variable.

The independent  variable is what scientists manipulate in the experiment. In the example radish hypothesis, the independent variable might be the concentration of magnesium in the nutrient solution. In good experimental design, it is important to select only one independent variable. For example, you should not vary both the magnesium concentration and the temperature conditions of your radish plants. If you did this, and got very tall radish plants, you would be unsure if the magnesium or the temperature was responsible for the change.  However, you would want to test at least three concentrations of magnesium in order to see if there is any kind of pattern with increasing magnesium concentration.  In summary, select one independent variable, and change it at least three times.

When a scientist selects an independent variable to manipulate, he or she will then try to control (keep constant) as many other variables as possible.  For this reason, these remaining variables, other than the selected independent variable, become controlled variables. In the radish example, if you selected magnesium concentration as your independent variable, you would try to control the temperature the radish plants are exposed to.  Temperature would become a controlled variable.  Other controlled variables might include light exposure, humidity, pH of solution, ambient noise, etc.

The dependent variable is what scientists measure, record, or observe in the experiment. Simple experiments have just one dependent variable.  In the example with the radish plants, the dependent variable could be the height of the radish plants.  However, unlike the strict requirements to just have one independent variable, there can be more than one dependent variable in an experiment. For example, you might measure length of leaves and weight of roots to assess the growth of radish plants. Selecting a dependent variable is sometimes restricted by the type and quality of tools that are available to measure the dependent variable.

If you are reading about someone else’s experiment, and trying to determine the independent, dependent, and controlled variables, the questions in the table below can help to make that distinction:

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