Elizabeth, a Licensed Massage Therapist, has a Master's in Zoology from North Carolina State, one in GIS from Florida State University, and a Bachelor's in Biology from Eastern Michigan University. She has taught college level Physical Science and Biology.
Scientists Ask Questions
Have you ever gone for a walk on a nice day and come across a pair of birds singing to each other? Did you wonder what they were saying? Maybe you've gone fishing with some friends and you became curious about why certain lures work better than others. Or perhaps you've wondered why when you spill a glass of water you have very little time to prevent it from going all over the place, but if you knock over a jar of peanut butter, you can easily reset it without losing a single drop.
Asking questions like this about our natural world is the mark of a good scientist. Scientists are constantly inquiring about why things are the way they are, how things work and how different things affect each other. They are incredibly curious about the intricacies of their surroundings, asking all sorts of questions about why, how, when, where and who.
The best way to answer these questions is with tests called experiments. These are step-by-step procedures used to validate hypotheses. Experiments usually provide evidence either supporting or refuting such hypotheses, but sometimes they are inconclusive and must be performed again.
For this reason, good experiments are designed so that they are repeatable. This means that if you, or someone else, wants to run your experiment again, you would be able to do so. This is important because we don't always find answers to our questions the first time, and being able to re-run the same experiment allows for the possibility of gathering more evidence.
Types of Variables
An important component of experiments is the different variables involved. A variable is any parameter in the experiment that can change. This could be a parameter like temperature, weight or height, or it could be more complex, such as life expectancy or growth rate. Depending on how you set up your experiment, you will use different kinds of variables. There are three basic types to be familiar with.
The first type is the independent variable. This is the variable that is manipulated during an experiment. For example, when experimenting with how fertilizer affects plant growth, the fertilizer is the independent variable because this is the variable that you change. It might help to think of the independent variable as the cause of change in the experiment.
In contrast to this, the dependent variable is the variable that is affected during an experiment. In our experiment, our dependent variable would be plant growth since this is affected by the amount of fertilizer applied. If you think of the independent variable as the cause, then the dependent variable is the effect.
If the independent variable is not the only factor that could affect the dependent variable, then you would have to make this factor a control variable. This is a variable that is controlled during and between experiments. In our fertilizer experiment, we would control for the amount of sunlight received by each plant. Control variables are very important because they allow a scientist to know that the experiment is only testing what they want to test. Depending on the complexity, some experiments will have many control variables, while others will have none.
Can you see how each type of variable gets its name? The dependent variable is dependent on the independent variable, and the independent variable is independent of the others. Control variables are controlled to make sure that the independent variable is the only factor causing a change in the dependent variable. Plant growth is likely affected by fertilizer (making it dependent), but fertilizer application is not affected by plant growth (making it independent). The amount of sunlight can affect growth rate, so it's controlled to stay the same in each experiment.
Let's look at a few different experiments to get a better idea of how these variables work. Say, for example, that you wanted to look deeper into that question you had earlier about fishing. The experiment you run depends entirely on the question you ask.
To start, you might ask, 'When is the best time of day to catch fish?' From past experiences, you hypothesize that fish are more active in the morning; therefore, this is the best time of day to catch fish. In your experiment, the time of day is the independent variable because this is what you change in the experiment. Dependent on this is the number of fish you catch, so this would be your dependent variable.
For this experiment, you will need to control for many factors, so you will have several different control variables. First, you need to make sure that you use the same equipment to catch fish each time. If you used different lures each time you went out, you would have no idea if the number of fish you caught was related to the time of day or the lure you used because both variables changed. This makes the fishing equipment one control variable.
You also need to make sure your experiment takes place in the same location each time. If you go fishing in Creek A during the morning and Creek B during the afternoon, you can't compare your catch from the two because there may be different numbers of fish in each location. You control for the difference between locations by fishing in the same spot each time.
Okay, so you run your experiment and you find that morning is in fact the best time of day to catch fish. But now your question is 'Where is the best place to catch fish?' To run an experiment for this question, you would need to hold the time of day constant (making it a control variable) while changing the location of where you go fishing each morning (making it the independent variable).
Car companies perform crash tests on their cars to determine how the cars and the people inside will be affected in an accident. Each time they perform a crash test, they are running experiments with different variables. For example, the car company may want to see how a certain car is damaged during head-on collisions at different speeds.
In this case, everything would be held constant except for the speed at which the car is smashed into the wall. They use the same model of car each time and make sure that each car weighs the same amount (both are control variables). This way, they can isolate how the speed (the independent variable) changes the amount of damage the car incurs (the dependent variable).
Because no two people are physically the same, the car company may also decide to see how a crash would affect different-sized drivers. For this experiment, they would hold the speed constant, making it a control variable this time. What changes is the size of the crash test dummy in the driver's seat. Because the size of the dummy is manipulated, this makes it the independent variable. And instead of the damage done to the car, the experiment is now testing the damage done to the dummy, making it the new dependent variable.
Scientists love what they do because they get to ask questions and then try and find the answers. Once a testable question has been formulated, scientists will conduct experiments to validate hypotheses in a step-by-step fashion.
A good experiment will be repeatable, and it will also have different types of variables. The independent variable is the variable that is manipulated during an experiment, and it gets its name because it is independent of other factors. It's the cause of the experiment that changes the dependent variable, which is the variable that is affected during an experiment. A control variable is a variable that is controlled during and between experiments, and it's used to make sure that the dependent variable is not affected by anything other than the independent variable.
The experiment you run depends entirely on the question you're asking, and your variables may change based on the experimental design. If you want to know where to catch the most fish, the location is the only variable that should be manipulated in the experiment to help you find an answer. However, if you want to know when to find the most fish, you've changed the question and the experiment, so the time of day becomes the variable that you manipulate.
Once the lesson is done you should be able to:
- Explain the purpose of an experiment
- Define and identify the independent, dependent and control variables in an experiment
- Formulate an experiment using these three variables
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