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Chapter Twelve. Designing and Conducting, Experiments with More than Two Groups. PowerPoint Presentation created by Dr. Susan R. Burns Morningside College. Experimental Design: Adding to the Basic Building Block. The two-group design is the basic building block.

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## Chapter Twelve

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**Chapter Twelve**Designing and Conducting, Experiments with More than Two Groups PowerPoint Presentation created by Dr. Susan R. BurnsMorningside College Smith/Davis (c) 2005 Prentice Hall**Experimental Design: Adding to the Basic Building Block**• The two-group design is the basic building block. • Researchers typically want to move beyond two-group designs so they can ask more complicated and interesting questions. Smith/Davis (c) 2005 Prentice Hall**The Multiple-Group Design**• How many IVs? • This chapter deals with experiments that use only one IV. • Although one-IV experiments are simpler than experiments that use multiple IV’s, they are not inferior in any way. • A well-designed experiment with one IV is vastly preferable to a sloppy experiment with many variables thrown together. Smith/Davis (c) 2005 Prentice Hall**The Multiple-Group Design**• How Many Groups? • This question marks the difference between the multiple-group design and the two-group design. • A multiple-group design compares three or more levels or amounts of an IV. • A multiple-group design can have a control group and two or more experimental groups. • We can compare three, four, five, or even more differing levels or amounts of an IV. • A multiple-group design does not have to have a control group. Smith/Davis (c) 2005 Prentice Hall**Psychological Detective**• Suppose you wished to test more than three noise conditions. Could you use a multiple-group design in such a case? Why or why not? Smith/Davis (c) 2005 Prentice Hall**The Multiple-Group Design**• Assigning Participants to Groups • After we decide to conduct a multiple-group experiment, we must decide about assignment or research participants to groups. • We may choose between independent groups or correlated groups. Smith/Davis (c) 2005 Prentice Hall**The Multiple-Group Design**• Independent samples • Groups of participants that are formed by random assignment. • Random assignment serves as an important control procedure. • One of several steps experimenters take to ensure that potential extraneous variables are controlled and avoid a confounded experiment. Smith/Davis (c) 2005 Prentice Hall**Random Selection**• Is Cathy using random assignment or random selection in this cartoon? • Because she is tasting chocolate Santas in a nonsystematic way (rather than assigning Santas to groups), Cathy’s gluttony illustrates random selection Smith/Davis (c) 2005 Prentice Hall**The Multiple-Group Design**• Correlated samples (nonrandom assignment to groups) • Matched sets • Participants are matched on a variable that will affect their performance on the DV (matching variable). • Then sets of participants are created who are essentially the same on the matching variable. Smith/Davis (c) 2005 Prentice Hall**The Multiple-Group Design**• Correlated samples • Repeated measures • Each participant must participate in all of the treatment conditions. • Natural sets • Analogous to using natural pairs except that sets must include more than two research participants. • Many animal researchers use littermates as natural sets. Smith/Davis (c) 2005 Prentice Hall**Deciding on an Experimental Design**• Researchers who want to design an experiment with one IV also need to choose between multiple-group designs and the two-group designs. Smith/Davis (c) 2005 Prentice Hall**Comparing Multiple-Group and Two-Group Designs**• All you have to do to change your two-group design into a multiple-group design is to add another level (or more) to your IV. • A two-group design can tell you whether your IV has an effect. Smith/Davis (c) 2005 Prentice Hall**Comparing Multiple-Group and Two-Group Designs**• A two-group design can tell you whether your IV has an effect. • You should never conduct an experiment to determine whether a particular IV has an effect without first conducting a thorough literature search • If you find no answer in a library search, then you should consider conducting a two-group (presence vs. absence) study. • A multiple-group design is appropriate when you find the answer to your basic question and wish to go further. Smith/Davis (c) 2005 Prentice Hall**Comparing Multiple-Group Designs**• Choosing a multiple-group design • Your first consideration should be your experimental question. • Then decide on whether to use independent or correlated groups. Smith/Davis (c) 2005 Prentice Hall**Comparing Multiple-Group Designs**• Control issues • The multiple-independent groups design uses the control technique of randomly assigning participants to groups. • If you have a substantial number of research participants (at least 10 per group), you can be fairly confident that random assignment will create equal groups. • Multiple-correlated groups designs use the control techniques of matching, repeated measures, or natural pairs to assure equality of groups and to reduce error variability. Smith/Davis (c) 2005 Prentice Hall**Comparing Multiple-Group Designs**• Remember the general equation that represents the formula for a statistical test • Reducing the error variability in the denominator of the equation will result in a larger computed statistical value, thereby making it easier to reject the Null hypothesis. • Using a correlated groups design reduces your degrees of freedom, which makes it more difficult to achieve statistical significance and reject the Null. • However, the reduced error variability typically more than offsets the loss of df. Thus, correlated designs often produce stronger tests for finding statistical significance. Smith/Davis (c) 2005 Prentice Hall**Comparing Multiple-Group Designs**• Practical considerations • Correlated-Groups Designs • Matched sets • You must consider the difficulty of finding three (or more) participants to match on the extraneous variable you choose. • Natural sets • May be limited by the size of the natural sets you intend to study. • Repeated measures • Each participant must be measured at least three times. • Independent groups Designs • You must take into account the large number of participants you will need to make random assignment feasible and to fill the multiple groups. Smith/Davis (c) 2005 Prentice Hall**Variations on the Multiple-Group Design**• Comparing different amounts of an IV • If we already know that a particular IV has an effect, then we can use a multiple-group design to help us define the limits of that effect. • In this type of experiment, we often add an important control in order to account for a possible placebo effect. • Placebo effect • An experimental effect that is due to expectation or suggestion rather than the IV. Smith/Davis (c) 2005 Prentice Hall**Variations on the Multiple-Group Design**• Dealing with measured IV’s • Ex post facto research deals with measured rather than manipulated IV’s. Smith/Davis (c) 2005 Prentice Hall

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