Correlation
Graphic Representation of Correlation
Inferential Statistics
Ethical GuidelinesExperimental Method
Some psychologists conduct experimental research in laboratories designed for carefully
controlling conditions and measuring behavior.The Controlled Experiment
The laboratory is one of the places where scientists test hypotheses,predictions of how two
or more factors are likely to be related. Variables are factors that can have different values.
In a scientific experiment,the researcher systematically manipulates a variable under con-
trolled conditions and observes the response. The factor the researcher manipulates is called
theindependent variable (IV).Thedependent variable (DV)is the behavior or mental
process that is being measured, the factor that may change as a result of manipulation of
the independent variable. If the dependent variable changes when only the independent
variable is changed, the researcher can conclude that the change in the independent variable
caused the change in the dependent variable. Thus, the independent variable is the cause,
and the dependent variable is the effect. A controlled experiment is the only research
method that can establish a cause and effect relationship.
An effective way to determine the independent and dependent variables is to word the
hypothesis in the form of an “If... , then.. .” statement. What follows the “if ” is the inde-
pendent variable (cause), and what follows the “then” is the dependent variable (effect). For
example, “If students study for a quiz before going to sleep, rather than in the morning,
then they will get higher scores on the quiz.” Studying for a quiz before going to sleep,
rather than in the morning, is the independent variable and cause. Getting a higher score
on the quiz is the dependent variable and effect.
For example, an experimenter hypothesizes that sleeping after studying for a biology quiz
in the evening is more effective than studying for the same amount of time after waking in the
morning. The populationincludes all of the individuals in the group to which the study applies
(all of the students enrolled in introductory biology courses at the university for this example).
To save time and money, most researchers use a subgroup of the population called a sample
in their experimental research. The larger the sample size, the more likely it is to represent the
population. The sample must fairly represent the whole group. This is achieved when every
member of the population has an equal chance of being selected for the sample, when partici-
pants are selected randomly. Random selectioncan be achieved by putting all of the names
in a hat and picking out a specified number of names, by alphabetizing the roster of enrollees
and choosing every fifth name, or by using a table of random numbers to choose participants.
These are examples. To test the hypothesis, the scientist needs to randomly assign some
subjects to an experimental groupthat receives the treatment and to randomly assign others
to a control groupthat does not receive the treatment. The control group is a comparison
group. This is called a between-subjects designbecause the participants in the experimental and
control groups are different individuals. Everything is similar between the experimental group
and the control group except for the independent variable. Random assignmentof participants
to the experimental and control groups minimizes the existence of preexisting differences
between the two groups. Differences between the experimental group and the control group
other than those resulting from the independent variable are called confounding variables.52 STEP 4. Review the Knowledge You Need to Score High“There’s a good
probability that
one of the essays
on the AP exam
will focus on
research methods,
especially
experiments.”
—Elliott S.,
AP Teacher
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