UNCW EDN 523 - Experimental Designs (13 pages)

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Experimental Designs



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1 EDN 523 Martin Kozlof Experimental Designs and Internal and External Validity This paper has three sections Section I provides a legend of the symbols used in describing experiments Section II describes pre experimental designs true experimental designs and quasi experimental designs Section III describes threats to internal and external validity It might be a good idea to skim Section III first I LEGEND X exposure of a group or an individual e g in a single subject design to experimental i e independent variables i e events conditions or treatments The experimenter may or may not have control over this exposure O observation or measurement Were the validity and reliability of instruments and measurement established prior to use Was observer reliability checked periodically during the experiment Left to right dimension time order X s and O x in a vertical line mean that observation and exposure to or change in experimental variables occur simultaneously R random assignment to comparison groups Was matching used to make groups more equivalent on certain variables Or was randomization used to give all possibly contaminating extraneous variables an equal chance to be in all groups Was the equivalence of comparison groups assessed pior to or after a pre test II DESIGNS PRE EXPERIMENTAL EXPERIMENTAL AND QUASIEXPERIMENTAL A Pre experimental Designs Pre experimental designs have either no comparison groups or comparison groups whose equivalence is indeterminate 1 2 1 One shot Case Study In this design there is almost complete absence of control over or at least a determination of extraneous variables that might account for the findings Since there is no pre test and no comparison group inferences that X had an effect are based upon a comparison between the one case studied and other cases observed and remembered That is a causal inference that X has an effect on the dependent variables observed is based on general expectations of what the data would have been had X not occurred X O 2 One Group Pre test Post test One group is exposed to the presence of X or a change in X and is measured before and after this has occurred The design at least enables you to compare the dependent variable s before and after X However the absence of a comparison group e g in design 4 or the absence of a series of changes in the independent variable s e g introducing and then removing an intervention several times in design 8 means that one cannot rule out plausible rival hypotheses explanations for differences between pre and post test e g maturation O1 X O2 3 Static group Comparison One group is exposed to X and is compared with another group which is not exposed to X However the absence of a pre test means that one cannot determine whether the groups were equivalent with respect to the dependent variable s or other extraneous variables X O1 O2 B True Experimental Designs These designs provide formal means pre tests and or comparison groups created by random allocation for handling many of the extraneous variables that weaken internal and external validity 2 3 4 Pre test Post test Experiment group Control group Design This is the classic design It allows comparisons within and between groups that are as similar as randomization can make them Note that the design is such that Mills Method of Difference can be used to infer a causal relationship between X and changes in dependent variables from pre to post test and differences between E and C groups at the posttest R R O1 O3 X O2 O4 Note however that the pre tests weaken external validity That is groups outside of the experimental situation may not normally be pretested 5 Solomon Four group Design This design formally considers sources of external invalidity especially the reactive effects of testing R R R R O1 O3 X X X O2 O4 O5 O6 Comparison of O2 and O4 suggests the effects of X but with the pretest Comparison of O5 and O6 suggests the effects of X without the pretest and enables you to assess the effects of the pre test Comparison of O2 and O5 also suggests effects of the pre test Comparison of O1 O6 O3 and O4 suggests the effects of the passage of time 6 Post test Only Control Group Design This is a useful design when pre tests are not feasible Note that randomization is an all purpose assurance of a lack of initial bias between the groups Compare with Designs 3 and 5 Is this design preferable to Design 4 because it handles the rival hypothesis of testing R X O1 3 4 R O2 C Quasi experimental Designs Quasi experimental designs lack control over exposure to X i e when to expose to whom and ability to randomize group assignment However they do have true experiment like features regarding measurement e g whom and when 7 Time series Design In this design the apparent effects of X are suggested by the discontinuity of measurements recorded in the time series The design is a moral alternative and probably just as strong as control group designs in which subjects are denied a potentiallybeneficial treatment O1 O2 O3 O4 X O5 O6 O7 O8 Compare this design with Design 2 History is the major weakness of this design which might be handled using control by constancy or a different design e g multiple time series 8 Equivalent Time samples Design This is a form of time series design with repeated introduction and removal of X with the same group or individual It is sometimes called an intra subject or intra group reversal or replication design Note the use of concomitant variation to establish the effect of X XO1 O2 XO3 O4 XO5 O6 XO7 9 Nonequivalent Control Group Design The groups involved might constitute natural collectivities e g classrooms that are as similar as availability permits but not so similar that one can dispense with a pretest as a way to determine equivalence Exposure to X is assumed to be under the experimenter s control O1 O3 X O2 O4 4 5 10 Counterbalanced Design This design enables you to assess the effects of numerous conditions e g treatments 1 2 3 4 by exposing each group to the treatments but in a different order Group Group Group Group A B C D X1O X2O X3O X4O X2O X3O X4O X1O X3O X4O X1O X2O X4O X1O X2O X3O A major problem with this design is multiple treatment interference That is Group A might show the most change during treatment 4 but this may be because Group A was previously affected by treatments 1 3 This possibility is partly handled by comparing Group A at condition 4 with the other groups where condition 4 happens earlier Even so it is


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