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Introduction Research Designthe outline, plan, or strategy used to answer the research question Research Designthe outline, plan, or strategy used to answer the research question Purpose of research design Purpose of research design –Control for unwanted variation –Suggests how data will be statistically analyzed

Designs with Threats to Internal Validity One-Group Posttest-Only Design One-Group Posttest-Only Design TreatmentResponse XYXYXYXY –Rarely useful because no pretest or control group –Almost all threats to internal validity apply –Is useful only when specific background information exists on the DV

One-Group Pretest-Posttest Design One-Group Pretest-Posttest Design PretesttreatmentPosttest YXY YXY –Most threats to internal validity exist –To infer causality must identify and demonstrate that internal validity threats do not exist

Nonequivalent Posttest-Only Design Nonequivalent Posttest-Only Design TreatmentPosttest Exper. Gp.XY Control Gp.Y Control Gp.Y –No assurance of equality of groups because they were not randomly assigned –May confound selection with treatment effect

Requirements of Experimental Research Designs Answer the research question or test the hypothesis Answer the research question or test the hypothesis Control for the effect of extraneous variables Control for the effect of extraneous variables –With control techniques (Randomization) –With a control groupgroup that does not get the IV or gets some standard value Serves as source of comparison to experimental group Serves as source of comparison to experimental group Controls for rival hypothesis Controls for rival hypothesis Should allow generalizability of the results Should allow generalizability of the results

Pretesting Participants To increase sensitivity through matching To increase sensitivity through matching To test for a ceiling effect To test for a ceiling effect To test for initial position To test for initial position To insure initial comparability To insure initial comparability To obtain evidence of change To obtain evidence of change

Experimental Research Designs Posttest-Only Design Posttest-Only Design TreatmentResponse TreatmentResponse Exper. Gp X Y Exper. Gp X Y Control Gp Y Control Gp Y –Represents basic components of most designs

Types of Posttest-Only Designs Between-participants Between-participants –Can also match participants prior to random assignment to insure equivalence on the matched variable Within-participants Within-participants

Types of Between-Participants Posttest-Only Designs Simple-Randomized Simple-Randomized –Used with more than 2 levels of one 1 IV –Participants randomly assigned to group Factorial Factorial –Used to test The effect of more than one IV The effect of more than one IV The interaction of several IVs The interaction of several IVs –Participants randomly assigned to groups

Simple-Randomized Posttest-Only Design TreatmentResponse TreatmentResponse Group I Y Group I Y Group IIX1Y Group IIX1Y Group IIIX2Y Group IIIX2Y Group IVX3Y Group IVX3Y

Factorial Design Factorial Design Independent Variable Independent Variable A1A2A3 A1A2A3 IndependentB1 IndependentB1 Variable BB2 Variable BB2 B3 B3

Components of a Factorial Design Cella treatment combination of two or more IVs Cella treatment combination of two or more IVs Main effectthe effect of one IV Main effectthe effect of one IV Interaction effectwhen the effect of one IV depends on the level of another IV Interaction effectwhen the effect of one IV depends on the level of another IV

Classic Interaction Effect Treatment Row Treatment Row A1 A2A3 Mean A1 A2A3 Mean B1 30 B1 30 B2 30 B2 30 Column Column Mean Mean

Difficulties with factorial designs Difficulties with factorial designs –Increases the number of research participants –Difficulty in interpreting higher-order interactions. Advantages of factorial designs Advantages of factorial designs –Can manipulate more than one IV –Can control potential extraneous variable by building it into the design –Provides greater precision when you add more than one IV –Can test the effect of interactions

Within-Participants Posttest-Only Design Treatment Treatment A1 A2 A3 A1 A2 A3 Same 20 P 1 P 1 P 1 Same 20 P 1 P 1 P 1 Participants in... Participants in... Each Treatment... Each Treatment... Condition P 20 P 20 P 20 Condition P 20 P 20 P 20

Advantages of within-participants design Advantages of within-participants design –Equivalence of research participants –Requires fewer participants than between- participants design Disadvantage of within-participants design Disadvantage of within-participants design –Sequencing effects

Pretest-Posttest Design Primary unique characteristic is that it includes a pretest in addition to a posttest Pre- test TreatmentPost-test Pre- test TreatmentPost-test Exper. Gp Y X Y Control Gp Y Y Control Gp Y Y