Scientists use experiments to search for cause and effect relationships in nature. In other words, they design an experiment so that changes to one item cause something else to vary in a predictable way.
These changing quantities are called variables . A variable is any factor, trait, or condition that can exist in differing amounts or types. An experiment usually has three kinds of variables: independent, dependent, and controlled.
The independent variable is the one that is changed by the scientist. To insure a fair test , a good experiment has only one independent variable. As the scientist changes the independent variable, he or she observes what happens.
The scientist focuses his or her observations on the dependent variable to see how it responds to the change made to the independent variable. The new value of the dependent variable is caused by and depends on the value of the independent variable.
For example, if you open a faucet (the independent variable), the quantity of water flowing (dependent variable) changes in response--you observe that the water flow increases. The number of dependent variables in an experiment varies, but there is often more than one.
Question Independent Variable (What I change) Dependent Variables (What I observe) Controlled Variables (What I keep the same) How much water flows through a faucet at different openings? Water faucet opening (closed, half open, fully open) Amount of water flowing measured in liters per minute
Water pressure, or how much the water is "pushing"
"Different water pressure might also cause different amounts of water to flow and different faucets may behave differently, so to insure a fair test I want to keep the water pressure and the faucet the same for each faucet opening that I test."
Does heating a cup of water allow it to dissolve more sugar? Temperature of the water measured in degrees Centigrade Amount of sugar that dissolves completely measured in grams
Type of sugar
"More stirring might also increase the amount of sugar that dissolves and different sugars might dissolve in different amounts, so to insure a fair test I want to keep these variables the same for each cup of water."
Question Independent Variable (What I change) Dependent Variables (What I observe) Controlled Variables (What I keep the same) Does fertilizer make a plant grow bigger? Amount of fertilizer measured in grams
Growth of the plant measured by its height
Growth of the plant measured by the number of leaves
See Measuring Plant Growth for more ways to measure plant growth
Same size pot for each plant
Same type of plant in each pot
Same type and amount of soil in each pot
Same amount of water and light
Make measurements of growth for each plant at the same time
"The many variables above can each change how fast a plant grows, so to insure a fair test of the fertilizer, each of them must be kept the same for every pot."
Does an electric motor turn faster if you increase the voltage? Voltage of the electricity measured in volts Speed of rotation measured in revolutions per minute (RPMs)
Same motor for every test
The motor should be doing the same work for each test (turning the same wheel, propeller or whatever)
"The work that a motor performs has a big impact on its speed, so to insure a fair test I must keep that variable the same."