AP Psychology

✪Correlation

✪Graphic Representation of Correlation

✪Inferential Statistics

✪Ethical Guidelines

Experimental 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 the independent variable (IV).Thedependent variable (DV)is the behavior or

mental process that is being measured, the factor that may change as a result of manipula-

tion of the independent variable. If the dependent variable changes when only the inde-

pendent 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

independent 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 morn-

ing, 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

TIP