Random assignment is used by research psychologists studying human behavior. In this lesson, you'll discover why random assignment is crucial to methods of experimentation and research design.
What Is Random Assignment?
Psychologists use experiments to investigate how manipulation of one factor causes a change in another factor. Scientists refer to these factors as one of two kinds of variables. The independent variable is that first factor: the one whose influence we're trying to measure. An independent variable doesn't change based on the other variables. The second factor - the one being influenced by changes - is called a dependent variable. This kind of variable changes based on the independent variable. Experiments are the best way to determine cause and effect relationships between these variables.
Psychologists rely on random assignment to assign subjects to different groups in an experiment. Random assignment leaves it completely up to chance to determine which subjects receive the critical part of the experiment, which is imperative for determining that the independent variable is indeed what creates the result. Randomly assigning subjects helps to eliminate confounding variables, or variables other than the independent variable that could cause a change in the dependent variable.
Suppose one day while studying for a test, you notice that you seem more focused and productive while you are listening to music. In fact, you think it's possible that listening to music while studying helps you earn better grades on tests. You have been taking psychology courses, and armed with the love of science, you decide to conduct an experiment to see if your hypothesis is correct.
You decide to test your hypothesis on the 300 students in your college introduction to psychology class. What is the independent variable in your experiment?
Remember that the independent variable is the part of the study that is manipulated or changed to determine a result. In your experiment, you will manipulate whether or not students listen to music while studying, so listening to music is the independent variable. The dependent variable then will be the subjects' scores on the test. The dependent variable shows the effect of the manipulation.
Experimental and Control Groups
To test the independent variable, you will need an experimental group and a control group. The experimental group is the group who receives the critical part of the experiment, the treatment. This is the group who will listen to music while studying.
But to know if the music has an effect on test scores, you will also need to compare the results of the experimental group to a control group, a group which doesn't receive the critical part of the experiment. In this case, our control group won't listen to music while studying.
How will you decide who is in the experimental group and who is in the control group? What about allowing students to choose which group they're in? No, that won't work. Maybe the students who choose to listen to music are already better students who excel at focused studying. We can't assume the results will be valid.
Okay, so how about picking the experimental group based on a first-come basis? Sorry, choosing to put the first 150 students who come to class in the experimental group is also not random assignment. Maybe those students who get up earlier to make it to class on time typically perform higher on tests because they get more sleep. Random assignment is the only way to eliminate other variables that could influence your results.
Making Random Assignment Happen
So how do you ensure random assignment? There are a lot of different methods; the only requirement is that every subject has an equal chance to be in the experimental group.
Drawing names out of a hat or creating a lottery are ways to make assignment to the experimental group random. Choosing every third name off a list of students is also random. Several computer programs can generate a random assignment of participants for you. The important piece is that subjects are equally likely to be in the experimental group.
Random assignment is the best way to assure that the only difference between the control group and the experimental group is whether or not they receive the treatment. Any other differences between groups, such as amount of sleep, G.P.A., or even unknown factors, are more likely to be equally distributed if subjects are chosen randomly.
Random assignment is the only way to assume the difference in test scores is caused by listening to music while studying. It allows psychologists, and budding psychologists like yourself, to have confidence that the results of your study are valid.
In psychology experiments, psychologists use random assignment to assign subjects to groups. Using random methods, subjects are assigned to either an experimental group, which will receive an experimental treatment and be observed, or the control group, which is observed under normal, non-experimental conditions. Random selection ensures that there will be no confounding variables, or factors that could influence the result of the experiment; confounding variables can render an experiment's findings completely invalid.
There are many ways to assign subjects completely randomly: You can use a computer program, take names from a list at regular intervals, or even pick names from a hat. The important thing is that a random assignment method must allow every subject an equal chance of being in either the experimental group or the control group.
- Random assignment helps to ensure that confounding variables do not occur
- To qualify for random assignment each subject must have an equal chance of being selected
- Random assignment allow researchers to be confident their findings are valid
After viewing this lesson, you should be able to:
- Define random assignment and other research method key terms
- Summarize the methods of a random assignment
- Provide examples of random assignments