In science, the term "simple experiment" has a specific meaning. To understand the differences between a simple experiment and a more complicated, controlled experiment, consider this example. Suppose that you are watching a candle flame and ask yourself, "Would the color of the flame be different if I added salt to the pool of melted wax at the base of the flame?" If you do this to only one candle, it is called a simple experiment. You can make it a better experiment, a controlled experiment, by using two candles of the same type. You light both, then add salt to the wax of one but not to the other.
After you have made your experimental change, you can now compare the two candles side by side. In the next chapter you will see how and why such controlled experiments can be much better science than simple experiments.
There are some special terms that will help us talk about different kinds of experiments:
- Experimenter (that's you)
- Independent variable The thing you are changing, in this case the salt you are adding to the wax of one candle. You, as the experimenter, make this change.
- Dependent variable The effect of the change, in this case how the color of the flame changed after you put the salt in the wax of one candle. This change depends upon the independent variable.
- Result Your observation of the change. In this case, what you observed about the change in color of the flame
If the flame does change color when you add the salt to the wax, we say that the independent variable and the dependent variable are related variables.
As an experimenter, you must try not to let other things change beside your two main variables. Let's say that you want to learn whether adding baking soda to the water used for watering bean plants will make them grow better. You must decide how much baking soda to put in the water and carefully observe the growth of the plants before and after giving them this water. Obviously, you must not make any other changes, such as adding more water, using different temperatures, or using a different amount of light. These variables would confound the results; you would not be able to judge which of the four variables—baking soda, or more water, or different light, or different temperature—might be related to any difference in growth. So you try to keep other possible variables constant, or the same, during the experiment. You try to keep all possible variables constant except the one you built into your experiment—adding the baking soda to the water as your independent variable.
Scientists use the term experimental design, or design of the experiment (or other investigation) to refer to different kinds of experiments. The design of the investigation is a most important part of doing good science. Professional scientists are always faced with the problems of the costs of an investigation, the amount of time they can devote to it, and other limits. You surely will be faced with similar limits as you do your investigation.
Many science projects displayed at science fairs cannot be seen as good investigations because the students simply did not put enough thought into their design. For example, you might see a project on growing plants that displays just one plant in each of two cups of soil. The experimenter's intention was to compare two different factors in the growth of the plants, such as a fertilizer ingredient or the color of the light. The big mistake in such limited investigations is the notion that the differences between just two such plants would be meaningful.
The fact is, the plants will differ anyway; no two plants are exactly alike. Therefore, the results are probably meaningless. The student doing such an investigation should consider investing much more thought, time, and materials in the project.
How many plants, or people, or animals should there be in such an investigation? For now, consider that 25 or more specimens should be used in each part of a controlled experiment for the results to be meaningful. Deciding about such numbers will get us into the study of what is called statistics. To some people, that term is a scary one, but it should not be. You are using statistics when you compare the scores of two teams in basketball, football, or other sports. You are using statistics when you look around a schoolroom and determine that there are more girls in the class than there are boys. You are using statistics when you judge that, at a certain age, girls on the average are taller than boys. Statistics simply means collecting and analyzing numerical data.
Statistics are a very important part of living as well as doing science, and you should learn how to be friendly with statistical concepts.
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