Experimental Designs

An experiment is a research method in which the investigator manipulates a variable(s) under very controlled conditions and examines whether changes occur in a second variable(s).

The manipulated variable is called the INDEPENDENT VARIABLE.  This is the presumed "cause."  The variable that is expected to change as a result of the manipulation of the independent variable is called the DEPENDENT VARIABLE.  This is the presumed "effect."

Try the following exercise to see if you can identify the independent and dependent variables.

EXERCISE

In an experiment, researchers are typically concerned about the performance of subjects in the experimental group.  If a researcher wants to know if a new drug helps improve memory, the researcher is most interested in the how people who are given the drug perform on the memory test.  However, in order to conclude that the drug "improves" memory, people who take it must perform better than those who do not take the drug.  The CONTROL GROUP serves as the BASELINE performance.  The group given the drug serves as the EXPERIMENTAL GROUP.
 

Extraneous Variables

In order to isolate the effect of the independent variable on the dependent variable, researchers must rule out alternative explanations.  In other words, only the independent variable can be allowed to vary.

The term EXTRANEOUS VARIABLE is used to refer to any other factor that might affect the dependent variable.

Try the following exercise to see if you can spot potential problems in these hypothetical research studies.

EXERCISE

To control for potential extraneous variables and other sources of error researchers use:

A standardized set of procedures

Equivalent Control and Experimental Groups

Standardized procedures means that subjects are treated the same way in all regards except for the independent variable(s).  Researchers also need to ensure that the control and experimental groups are similar on important variables at the outset.  To do this researchers can use one of three methods.

• Use the same subjects in both the control and experimental groups.  (This is called a repeated measures design).

 
• Match subjects on important variables (e.g., for every 20 year old female in the control group there is a 20 year old female in the experimental group).

• Random assignment. (Let chance decide who gets placed into which group.  Thus, each subject has an equal chance of being placed in either group).

 
Why three methods?

Sometimes we cannot use the same subjects in both the control and experimental groups.  Sometimes after having been in one of the conditions it alters the subjects' behavior.  This change may carry over to the next condition and thus serve as an extraneous variable.

For example, a researcher wants to study whether a new drug is better than an old drug to reduce anxiety symptoms.  If we gave the old drug to the subjects and assessed them and then gave the new drug, there might be carry-over effects from the old drug still.  Thus, we might want to use two different groups of people who suffer from anxiety.

We could match our subjects on important variables such as age, gender, severity of symptoms.  Thus, for every 40 year old male with mild symptoms in the old drug (control) group there is a similar subject in the new drug (experimental) group.  However, in finding perfect matches for our subjects we might have to go through many people.  This is not very resource efficient.  In order to find 50 people who are perfect matches for another group of 50 we might have to go through a few hundred potenital subjects.

A more simple method is random assignment.   We let chance determine who is in the control and experimental groups.  With a large enough sample of subjects it is highly unlikely that the majority of people with severe symptoms would be in one group.
 

Advantages & Disadvantages of the Experiment

The advantage of the experimental approach is that it allows investigators enough control to examine cause and effect relationships.   Experiments allow us to answer "what causes something to occur?"  This is the second goal of science, understanding and prediction.
 

However, this degree of control can also be a potential weakness for experiments.  By controlling features of the environments of subjects the researcher may create too artificial an environment.  This means that while the researcher may have accurately understood the "cause" of the subjects' behavior, the findings only apply under such rigid, non-real world conditions, to have limited use in explaining real-world behavior.  (The desire of psychology is to understand this real-world behavior too).

A second weakness for experiments is that some questions for ethical or technical reasons cannot be studied using an experiment.  An important question is whether people who have had less optimal rearing experiences, such as poverty or abuse, continue to have difficulties in their adult years because of this poor rearing.  Yet, we cannot place children in abusive environments just to see if it "causes" damage that persists into adulthood.  Thus, we use other research methods, such as correlational studies.  We might see whether there is a relationship between childhood poverty or abuse and psychological and behavioral problems of adults, by asking adults about their childhood experiences and their life as an adult.
 

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 ©svf 03/27/03