One simple example would be setting up an experiment to see if a particular plant food improved plant growth over a month. First, we would get 20 of the same type of plants of a similar age and size. In this example, plant type, age, and size at the start of the experiment are variables that we want to keep the same. Next, we would divide the plants into two groups by numbering them and randomly drawing numbers. The first 10 plants would go into our treatment group—the plants that get the plant food. The second 10 plants will go into what we call our control group—these are the plants that do not get plant food. This method of assigning things to groups is called randomization, and it makes sure that every plant involved in our study has an equal chance of being picked for either group. This is the best way to make sure that the groups are as equal as possible. Then, for a month, we give the plant food to the treatment group but not to the control group. All the other variables are kept exactly the same—the plants get the same amount of sunlight, the same environment, and the same water.

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