Biology-Mr.Willets
Single Variable Experiment
A single variable experiment is one in which the experimenter (you) changes only one factor (the single variable) in order to see its effect on the experimental subjects. To do this, there must be two groups of subjects. These two groups must be as similar as possible. It is easier to get two groups to be similar if large numbers of subjects are used (the more, the better, within limits of space etc.) Large groups are also helpful because if there is one unusual member of the group, it will not affect the average all that much.
These two groups are treated exactly the same except for the single variable (also called the independent variable). The group that is treated normally is called the control group. The group that is treated differently is called the experimental group. This type of experiment is also called a controlled experiment. If, at the end of the experiment, these two groups are somehow different, the difference must have been caused by the single variable. (Why couldn't this difference have been caused by something else?)
Usually, you have an idea about how the variable may affect the groups. If so, then you expect the experimental group to turn out differently in a specific way. It is this difference that you will look for and measure as the experiment goes on. The way in which the experimental group becomes different from the control group is called the dependent variable (because it was caused by or depends on the independent variable)
Before starting the experiment, you create a hypothesis. This is a statement of how you think the independent variable will affect the dependent variable. An example- "Water is needed for plants to grow normally." The independent variable will be water (the control group will be watered normally, the experimental group will not be watered) The dependent variable is growth (you believe that the growth of the plant depends on the amount of water). You will measure growth during the experiment and you expect that lack of water will slow down or stop growth. A hypothesis can be thought of as a prediction based on observations, research and sometimes just common sense.
The biggest problem with this type of experiment is making sure that both groups are different in only one way. If, for example, you don't water one group of plants and they are also in a shadier spot than the watered group, you will not be sure whether the lack of water or the lack of sun (or both) caused them to grow differently. This is an especially tough problem when experimenting with people since no two groups of people will be all that similar and you cannot control everything people do. The only way around this is to use lots of people and do the experiment as many times as possible and hope that any differences that you cannot control will even out over the long run.
When the experiment is completed, your data leads to a conclusion. This is what you learned as a result of the experiment. Sometimes the experiment shows your hypothesis to be true. If so, the conclusion will be the same as the hypothesis. Using the example above- "Plants will not grow as well if they are not watered." or "Water is needed for plant growth." Sometimes your experiment disproves your hypothesis in which case your conclusion will be the opposite of your hypothesis. Example- "Water does not affect the growth of plants." or "Plants grow better without water." Sometimes your experiment is inconclusive. This means that you cannot come up with a definite conclusion. When this happens, you often try again, changing your experiment in some small way.
Running a single variable experiment with people always is tricky because people know they are part of an experiment and that may affect the results. If, for example, you are testing a new drug, the people who get the drug will have a different attitude than the people who are not getting any drug. This difference in attitude might affect the results. Also, the person running the experiment might treat the people in one group differently than the people in the other group. This might also affect the results. To avoid this, nobody can know who is in which group. This is called a "double blind" experiment. The people in the groups don't know which group they are in and the person running the experiment doesn't know who is in which group. They are both unaware (blind) about the groups. To make this work, the group not getting the drug must be given a phony pill called a placebo. Only when the experiment is over, does everybody find out who was in which group.
1)Fill in the blanks about this experiment:
I have noticed that the grass growing under trees does not grow as fast as the grass in the sun. I propose the following ____________________: " Grass grows better when it receives more sunlight"
I then use two containers to grow some grass. One container I put in the sun, the other I leave indoors.
The amount of light is the ________________________________. How fast the grass grows is the _______________________________. The grass in the sunlight is the __________________ group. The grass grown indoors is the ______________________ group. Because I used hundreds of grass seeds in each container, it is likely that the two groups will be very _________________________.
Give some examples of what I must do to make sure that this is a single variable experiment
2) Why do we use the word "dependent" in the phrase "dependent variable"?
3) In an experiment, if one group turns out differently from the other, how do we know the difference was caused by the independent variable and not something else?
4) Using large groups in an experiment is helpful in two 2 ways- a) It is easier to make sure the two groups are ______________________________ b) If there is one member of the group that is ____________________ in some way, it will not affect the average of the group.
5) It is hard to do a controlled experiment with people because a) getting two groups of people that are the ________________ is difficult b) you cannot control everything that _____________________
6) Because people know that they are _________________________________, this can affect the way they act and the results. Therefore, people in the control group are given a _______________________ so no one knows who is in which group.
The person running the experiment cannot know who is which group because he might sub-consciously treat them differently. This type of experiment is called a ___________________________ experiment.