Chapter 8 ___________________________________ Experimental Design - - PDF document

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Chapter 8

Experimental Design

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Intro:

True experimental method, all extraneous variables are controlled. Create groups that differ in the levels of the dependent variable. Variables are kept constant through direct control or randomization If scores of the two groups are different, then the experimenter can reasonably assume the difference is due to the manipulated variable.

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Confounding Variables-one which varies along with the independent variable…confounding the results. Internal validity-When the results of an experiment can confidently be attributed to the effect of the I.V.

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Basic Experiments

Post-test only Pre-test/Post-test Design Independent Groups Design Repeated Measures Design

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Post-test Only

Participants are assigned to at least two groups, experiment and control  the effect is measured

• nce.

Levels of independent variable are chosen: two levels at least  treatment and control, or two levels of treatment—can have more than one level. Advantage: easy & inexpensive Disadvantage: Does not always assure groups were truly equivalent (experimental & control)

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Pretest/Posttest

a pre-test is given before the manipulation is introduced. A pre-test can assess whether the groups were equivalent prior to the manipulation.. Pre-test allows the measurement of the extent of change in the group or an individual participant. Advantage: Can assess equivalence, mortality and extent of change. Disadvantage: Time consuming and awkward., can sensitize the participants to what you are looking for and they may act differently.

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Solomon’s Four Group Design

ability to directly assess the impact of the pretest with a combination of both the posttest-only and the pretest-posttest design.

 Half of the participants receive only the posttest

and the other half receive both the pre and post test.

 If there is on impact of the pretest, both of the

posttest scores will be the same.

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Independent Groups Design

Different participants are assigned to each

• f the condition using random assignment.

Decision to assign an individual to a particular condition is completely random and beyond the control of the researcher.

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Repeated Measures

Alternative to the independent groups design  the same individual participants are in all of the groups. Advantages:

 Fewer research participants needed as each participant

serves in all conditions.

 Extremely sensitive to finding statistical significant

differences

 Reduces random error due to individual differences as

there is data from the same people in both conditions

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Repeated Measures

Disadvantages:

 Order effects-different conditions must be

presented in a particular sequence which can cause two types of problems

 Practice effects-an improvement in performance

as a result of repeated practice.

 Fatigue effects-a deterioration in performance as

the research participant becomes tired, bored or distracted.

 Carry-over effects-effect of the first treatment

carries over to influence the response to the second treatment.

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Approaches to Order Effects

Counterbalancing-all possible orders of presentation are included in the experiment . Latin square-is a limited set of orders constructed to insure that each condition appears at each ordinal position, and that each condition precedes and follow each condition

• ne time.

Randomized Blocks-basic experimental procedure is repeated many times. Time Interval Between Treatments

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Other Research Designs

Matched Pairs Design Developmental Research Designs

Cross Sectional Method Longitudinal Method

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