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Chapter 8. Experimental Design. 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
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Chapter 8 Experimental Design
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.
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.
Basic Experiments • Post-test only • Pre-test/Post-test Design • Independent Groups Design • Repeated Measures Design
Post-test Only • Participants are assigned to at least two groups, experiment and control the effect is measured once. • 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)
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.
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.
Independent Groups Design • Different participants are assigned to each of the condition using random assignment. • Decision to assign an individual to a particular condition is completely random and beyond the control of the researcher.
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
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.
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 one time. • Randomized Blocks-basic experimental procedure is repeated many times. • Time Interval Between Treatments
Other Research Designs • Matched Pairs Design • Developmental Research Designs • Cross Sectional Method • Longitudinal Method