CHAPTER 8 EXPERIMENTAL DESIGN.

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Presentation transcript:

CHAPTER 8 EXPERIMENTAL DESIGN

LEARNING OBJECTIVES Describe the relationship between a confounding variable and the internal validity of an experiment Describe the posttest-only design and the pretest-posttest design, including the advantages and disadvantages of each design

LEARNING OBJECTIVES Contrast and compare independent groups design with a repeated measures design Summarize the advantages and disadvantages of using a repeated measures design Describe a matched pairs design, including reasons to use this design

CONFOUNDING AND INTERNAL VALIDITY Confounding Variable Another variable that occurs along with the independent variable Is an uncontrolled variable Cannot determine which variable is responsible for the effect

CONFOUNDING AND INTERNAL VALIDITY An experiment must be designed and conducted so that only the independent variable can be cause of the results Internal validity exists when the results of an experiment can be confidently attributed to the effect of the independent variable

BASIC EXPERIMENTS Posttest-Only Design Must: Obtain two equivalent groups of participants Care must be taken to eliminate selection differences Introduce the independent variable Measure the effect of the independent variable on the dependent variable

POSTTEST-ONLY DESIGN © 2009 The McGraw-Hill Companies, Inc.

BASIC EXPERIMENTS Pretest-Posttest Design A pretest is given to each group prior to introduction of the experimental manipulation Assures that groups are equivalent at the beginning of the experiment Can quickly measure changes that occur from the pretest to the posttest

BASIC EXPERIMENTS Advantages of the pretest-posttest design Mortality (dropout factor) Assess equivalency of groups with small sample size Can be used to select participants for the experiment

BASIC EXPERIMENTS Disadvantages of the pretest-posttest design Time consuming and awkward to administer Sensitizes participants to what is being studied Demand characteristics Reduces external validity

© The McGraw-Hill Companies, Inc.

ASSIGNING PARTICIPANTS EXPERIMENTAL CONDITIONS Independent Groups Design Participants participate in only one group Repeated Measures Design Participants are in all conditions

REPEATED MEASURES DESIGN Advantages and Disadvantages of Repeated Measures Design Advantages Fewer participants Extremely sensitive to statistical differences Conditions are identical because person is own control group Disadvantages Order effect Practice effect Fatigue effect Carryover effect

REPEATED MEASURES DESIGN Counterbalancing Complete counterbalancing Latin squares Time Interval Between Treatments Choosing Between Independent Groups and Repeated Measures Design

COUNTERBALANCING © 2007 The McGraw-Hill Companies, Inc.

LATIN SQUARE WITH FOUR CONDITIONS

MATCHED PAIRS DESIGN Goal is to Match People on a Participant Characteristic Matched to either the dependent measure or a variable that is strongly related to the dependent variable