A controlled experiment is an experiment that is conducted under a setting that is “controlled” by the scientist with the least possible influence from outside factors. A controlled experiment compares an experimental group with a control group in the exact same conditions with the one exception of a special/different treatment given to the experimental group.
The control group represents the "normal", "standard" or untreated condition. The experimental group is the one part of the experiment that is changed/manipulated (given different treatment). The constants are everything else you keep the same to ensure a fair test.
Francesco Redi's Controlled Experiment: Do maggots (young flies) come from rotting meat?
To better understand controlled experiments, let's look at Francesco Redi's famous controlled experiment on maggots and meat as an example. Francesco wanted to find the answer to the question, "Do maggots (flies) come from rotting meat?". This may seem like a really silly question, but in Francesco Redi's day (1600s), people actually believed that flies and maggots were generated spontaneously from meat. Take a look at Redi's experiment below.
Louis Pasteur also conducted a famous controlled experiment. His experiment addressed the question, "Can microorganisms (germs) generate spontaneously?" For hundreds of years before Louis Pasteur, scientists believed that microorganisms (living things too small to see with the naked eye) came from thin air. Below is Pasteur's experiment.
A controlled experiment is usually the fairest and most accurate way for a scientist to gather evidence to test a claim (hypothesis). Therefore, we will be using controlled experiments, like Francesco Redi and Louis Pasteur, in science class this year as we develop scientific explanations of phenomena that interest us.
A controlled experiment can be replicated
Another important thing to note about controlled experiments is that they are super easy to replicate (copy). Therefore, any scientist should be able to gather the same evidence and results when conducting the same experiment. In other words, if you performed Francesco Redi's famous experiment from the 17th century today, you would get the exact same results (replicated results)!
Limitations of a controlled experiment
However, keep in mind that controlled experiments have their limitations. First of all, not every experiment can be controlled in a laboratory setting. For example, if you want to study things in nature, such as how specific plants and animals interact in the rain forest, there are many factors that you would not (and should not) be able to control/change. Furthermore, controlled experiments do not necessarily represent real-world conditions, and can therefore create unrealistic results. For example, if a scientist is able to prove the effectiveness of a new drug (medicine) in a controlled laboratory setting, this does not necessarily mean that the drug will be as effective among people who have very different diets and live in very different environments.
Perform Redi and Pasteur's famous controlled experiments here! Then answer the questions below.
1. What is a controlled experiment?
2. What are the two "groups" in a controlled experiment?
3. What is a control group?
4. What are constants?
5. Identify the experimental and control group in Francesco Redi's controlled experiment.
6. How was Louis Pasteur able to gather evidence to prove that microorganisms do not generate spontaneously? Explain. If you need help, review Louis Pasteur's experiment here.
7. Why do you think it is important to keep all conditions in a controlled experiment the same except the one special/different treatment?
8. Why is it important that you can "replicate" a controlled experiment?
9. What is one limitation of a controlled experiment?
Watch the video below on a controlled experiment studying the effect of ozone on plants Then answer the questions in your science notebook in complete sentences.
1. What was the question the scientists were trying to answer through this controlled experiment?
2. How were all of the plants treated the same?
3. What is a variable?
4. What is the variable (one thing changed) in this experiment?
5. What was the control/control group in this experiment?
6. What is a control/control group?
7. What do the scientists measure/record everyday in this experiment?
8. Why do you think it is important that scientists only test one variable in an experiment?
Watch the video of how students set up a controlled experiment to investigate the effect of mirrors on dolphin behavior: http://vitalny.pbslearningmedia.org/resource/737ed8f2-10bb-484e-9b75-e3f19a91f3a4/dolphin-dive-05-test/
Then answer the questions below.
1. What was the control test/group in the dolphin experiment?
2. What was the experimental group in the dolphin experiment?
3. How do you think the students could have improved their experiment?