Free 5 Steps to a 5 AP Psychology, 2010-2011 Edition by Laura Lincoln Maitland

be related. Variables are factors that can have different values. In a scientific experiment , the researcher systematically manipulates a variable under controlled conditions, and observes the response. The factor the researcher manipulates is called the independent variable (IV) . The dependent 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.
“ 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

    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 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 population includes 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 participants are selected randomly. Random selection can 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 group that receives the treatment and to randomly assign others to a control group that does not receive the treatment. The control group is a comparison group. This is called a between-subjects design because 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 assignment of 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 .Confounding variables limit confidence in research conclusions. All participants, also called subjects , attend the same two sessions upon which the quiz is based. The experimental group is permitted to study for the quiz for 1 hour in the evening before going to sleep while the control group watches an unrelated comedy show. The control group studies for the quiz for 1 hour in the morning after awakening. The experimental group watches the comedy show in the morning at the same time. Everyone eats breakfast together, then