The Experimental Method in Psychology

Experimentation Experiment: a research method in which an investigator manipulates one or more factors (variables) to observe the effect on some behavior or mental process (the dependent variable).

Experimentation Experiment: Can isolate cause and effect Control of factors Manipulation of the factor of interest (experimental group) Hold constant factors (control group)

Experimentation Random assignment: Participants are assigned to groups by chance, thus minimizing any differences between those assigned to the different groups. Eliminates alternative explanations Different from random sample

Experimentation Blind assignment (un-informed) Single-Blind Procedure Participants do not know which group they are assigned to Double-Blind Procedure Neither participants nor the research staff know which group the participants are assigned to.

Experimentation Placebo effect: Experimental results that are caused by expectations alone; any effect on behavior caused by the administration of an inert substance or condition

Experimentation Independent and Dependent Variables Independent Variable (IV): The experimental factor that is manipulated; the variable whose effect is being studied. Dependent Variable (DV): What is being measured. In other words, the variable that may change in response to the IV

Experimentation Independent and Dependent Variables Confounding Variable: A factor other than the independent variable that might produce an effect on the experiment

Experimental Design Question: Do children who are breast-fed as infants grow up to be smarter than those fed on a bottle?

Experimental Design Hypothesis: Population: all children Children who are breast-fed as infants score higher on intelligence tests than those who are bottle fed. Population: all children

Experimental Design

Experimental Design

Experimental Design

Experimental Design

Comparing Research Methods Basic Purpose How conducted What is manipulated Descriptive To observe and record behavior Case studies, surveys, naturalistic observation Nothing is “changed”

Comparing Research Methods Basic Purpose How conducted What is manipulated Correlational To detect naturally occurring relationships (assess how well one variable predicts another) Computing statistical associations Nothing is manipulated

Comparing Research Methods Basic Purpose How conducted What is manipulated Experimental To explore cause and effect Manipulate one or more variables Use random assignment to eliminate preexisting conditions The independent variable(s)