5 Steps to a 5 AP Psychology, 2010-2011 Edition - Laura Lincoln Maitland [28]
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 they all take the same quiz. If the experimental group scores significantly higher than the control group, the experimenter can say that the results support the hypothesis. How does the experimenter measure effectiveness of studying? The experimenter uses the score on the quiz as the operational definition of effectiveness of studying. An operational definition describes the specific procedure used to determine the presence of a variable.
In order to attribute a particular result to a specific factor, the controlled experiment must limit variables. Confounding variables that could contribute to the effect must be eliminated. Subjects in the biology quiz experiment need to share the same environmental factors; they need to eat the same foods, sleep in similar beds in the same rooms, sleep for the same amount of time, etc.
Eliminating Confounding Variables
Experimenter bias (also called the experimenter expectancy effect) is a phenomenon that occurs when a researcher’s expectations or preferences about the outcome of a study influence the results obtained. This is a special kind of confounding variable that can occur when a researcher is unaware that he or she is treating either the experimental group or control group differently from the other. A simple smile when