Controlled Experiments

Meet Mark and Molly. They were just given some plants. The only problem is that neither Mark nor Molly has a green thumb, they are not very good at growing plants. In order the keep the plants healthy Mark and Molly need to figure out how much to water the plants.

Mark takes his plant, places it in the window and decides to give it 100ml of water once a week. Molly hides her plant from Mark and places it in the closet. There she decides to water it with 100ml of water three times a week.

One month later.

Mark's plant is green, tall and healthy. Molly's is looking a little yellow and not so healthy. Mark concludes that it is better to water a plant once a week. Although Mark's plant is healthier something was not fair in this comparison. If you remember Mark's plant received sunlight while Molly's did not.

To figure out how much to water their plants Mark and Molly needed to set up a controlled experiment. In this type of experiment everything needs to stay the same except one variable. Both plants need to be planted in the same size pot, using the same type of soil and the same amount of soil. Each plant needs to be in the same temperature and what Mark and Molly did not do was give each plant the same amount of sunlight. Then one part of the experiment can be manipulated. In our example the amount of water used to water the plants is the only thing different between the two plants. The part of the experiment that is changed is called the independent variable.


One month later.


The results are different this time. Given equal amounts of sunlight we see that the plant responded better to Molly's amount of water. To ensure the accuracy of the experiment Mark and Molly could repeat it a few times to make sure they did not make any mistakes.

Controlled experiments are used in science all of the time. Just remember to keep everything constant or the same except one variable that you change.