EXPERIMENTAL DESIGN1

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

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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

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Confounding Variable Another variable that occurs along with the

independent variable Is an uncontrolled variable Cannot determine which variable is responsible for

the effect

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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

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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

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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

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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

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Disadvantages of the pretest-posttest design Time consuming and awkward to administer Sensitizes participants to what is being studied

- Demand characteristics- Reduces external validity

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Independent Groups Design Participants participate in only one group

Repeated Measures Design Participants are in all conditions

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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

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Counterbalancing Complete counterbalancing Latin squares

Time Interval Between Treatments Choosing Between Independent Groups and

Repeated Measures Design

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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

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