Chapter 11: The Nuts and Bolts of one-factor experiments

Experiments Experiment: a type of research design that involves manipulation of an independent variable, allowing control of extraneous variables that can affect the results. Small-n design: an experiment conducted with one or a few participants to better understand the behavior of those individuals. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Causation Through Manipulation and Control Independent variable: a variable in an experiment that is manipulated by the researcher such that the levels of the variable change across or within subjects in the experiment. A participant’s gender, personality, and smoking status cannot be independent variables. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Causation Through Manipulation and Control Third-variable problem: the presence of extraneous factors in a study that affect the dependent variable and can decrease the internal validity of the study. Must be considered in all studies. Confounding variable: an extraneous factor present in a study that may affect the results. Time of day experiments are run, slight alteration in instructions given by different experimenters, etc. The researcher must design an experiment to control for the confounding variables as they may affect the results. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Causation Through Manipulation and Control Between-subjects variable: each participant experiences only one level of the variable. A researcher interested in the effects of text format on learning can manipulate text format as a between-subjects variable by randomly assigning different students in a class to a hard copy version of the text and an online version of the text. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Causation Through Manipulation and Control Within-subjects variable: each participant experiences all levels of the variable. Random assignment: participants are randomly assigned to levels of the independent variable in an experiment to control for individual differences as an extraneous variable. Subjects are assigned to groups according to chance. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Causation Through Manipulation and Control Order effects: occur when the order in which the participants experience conditions in an experiment affects the results of the study. Learning can be different depending on whether the participants receive the hard copy version or the online version of the text first. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Causation Through Manipulation and Control Testing effects: occur when participants are tested more than once in a study, with early testing affecting later testing. Order effects are a particular form of testing effects. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Between-Subjects Experiments The presence of individual differences across groups is the greatest concern for between- subjects experiments. In experiments where individual differences are likely to be present and a small sample size is used, random assignment may not be sufficient to control for these differences. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Between-Subjects Experiments A matched design can be used to control for differences. In a matched design, participants are matched on a characteristic that may contribute to group differences. Then each participant within a matched set is randomly assigned to different groups. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Between-Subjects Experiments One way to ensure that participants in the groups are matched on some characteristic is to measure that characteristic (e.g., years of education, socioeconomic status, language abilities) during the experiment and compare the groups in an additional analysis. This comparison will indicate if the groups are similar or different on that characteristic. However, if the groups are different, you are then left with the possibility that this difference affected your results. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Within-Subjects Experiments The primary concern of within-subjects variables is the order in which the participants receive the different levels of the independent variable. Because the participants receive all levels of the independent variable, it is possible that order of the levels can affect the dependent variable measure. For example, suppose that a within-subjects variable in an experiment involves a task difficulty manipulation such that all participants receive both an easy task and a difficult task. The order in which they receive these tasks can certainly affect the outcome of the study. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Within-Subjects Experiments To solve the problem of order effects for within- subjects manipulations, a researcher will typically counterbalance the order of the levels within the study. This means that different groups of participants will receive different orders of the levels of the independent variable. For example, half of the participants would receive the easy task first, and half would receive the difficult task first with participants assigned in equal numbers to the two different orders. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Within-Subjects Experiments A partial counterbalancing technique, called a Latin Square, can be used. In a Latin Square, the number of orders used in the experiment is equal to the number of conditions in the design, and each condition is in each ordinal position (i.e., first, second, third, fourth) exactly once. Latin Squares can be useful for within subjects designs where independent variables have the same number of levels. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Experiment Examples Cognitive example Biological example Social example McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.