Smith/Davis (c) 2005 Prentice Hall Chapter Twelve Designing and Conducting, Experiments with More than Two Groups PowerPoint Presentation created by Dr. Susan R. Burns Morningside 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.