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Variables Tip Sheet In a scientific experiment, you are measuring how changing one factor in a system affects another factor. These factors are called variables, and an experiment generally has three types: independent, dependent and controlled.

Tip: Good variables are usually those that you can measure with a number (e.g., concentrations of pollutants, time of day, temperature) or observe objectively (e.g., colors, growth).

The independent variable is the one that you, as the scientist, change or manipulate. Your experiment should have only one independent variable. For your EnvironMentors project, possible independent variables could be time (e.g., testing the amount of pesticides in the water at the same site on different days) or location (e.g., measuring noise pollution at different locations at the same time of day). Time and location are just two examples of independent variables. You and your mentor should work together to determine the best independent variable for your project.

The dependent variable changes in a consistent manner in response to changes you make in the independent variable. In the examples above which used time and location as the independent variables, the dependent variables would be the amount of pesticides in the water, and the amount of noise pollution, respectively. Whereas there should only be one independent variable, the number of dependent variables can vary. For instance, if you were investigating water quality at the same site on different days, your independent variable would be time, and each different pesticide concentration you measure in the water would be a different dependent variable.

The controlled variables are those that you try to keep constant throughout your experiment.

These are all variables that might affect your dependent variable. If you were measuring the concentrations of pesticides in water on different days, you would want to make sure that things like the weather and outside temperature remain relatively constant. You should keep track of your controlled variables as carefully as your other variables. Remember that it is important to try to keep your controlled variables as constant as possible. For instance, if you measured the noise pollution at two different traffic intersections in your city (independent variable = location; dependent variable = noise pollution) but made your measurements at different times of day, you wouldn’t be able to tell if changes in the amount of noise pollution were due to the different location or the different time of day. Most experiments will have more than one controlled variable. Talk to your mentor and make a list of all the variables you think might affect your dependent variable, and the best way to make sure you keep those variables constant.

You must also compare an experimental group to a control group. The control group is the set of experimental trials where the independent variable is set at its natural state. An experimental group is the set of experimental trials where you change the independent variable.

Example: In an experiment investigating the effects of acid rain on plant growth, you might grow different plants and water them with water at different pH values. The plants watered with water with a pH natural� rain value of 5.6 would be your control group and the plants watered with water with more acidic pH values would be your experimental group. In some projects, it might not be possible to measure a control group. Be sure to discuss this with your mentor and/or chapter coordinator.