Chapter Fourteen

Chapter Fourteen Designing and Conducting Experiments with Multiple Independent Variables PowerPoint Presentation created by Dr. Susan R. BurnsMorningside College Smith/Davis (c) 2005 Prentice Hall

Experimental Design: Doubling the Basic Building Block • A factorial design gives us the power we need to devise an investigation of several factors (IVs) in a single experiment. Smith/Davis (c) 2005 Prentice Hall

Experimental Design: Doubling the Basic Building Block • Factors • Synonymous with IVs • Independent variables (IVs) • Stimuli or aspects of the environment that are directly manipulated by the experimenter to determine their influences on behavior. Smith/Davis (c) 2005 Prentice Hall

Experimental Design: Doubling the Basic Building Block • Factorial designs are the lifeblood of experimental psychology because they allow us to look at combinations of IVs at the same time, a situation that is quite similar to the real world. • A factorial design is more like the real world because there are probably few, if any, situations in which your behavior is affected by only a single factor at a time. Smith/Davis (c) 2005 Prentice Hall

Experimental Design: Doubling the Basic Building Block • This figure demonstrates a graphical display of the simplest possible factorial design (2 x 2). • This 2 X 2 shorthand notation tells us that we are dealing with a design that has two factors (IV’s) because there are two digits given and that each of the two factors has two levels because each digit shown is a two. Smith/Davis (c) 2005 Prentice Hall

How Many IV’s? • The factorial design gets its name because we refer to each IV as a factor. • Multiple IV’s yield a factorial design. • Theoretically, there is no limit to the number of IV’s that can be used in an experiment. • Practically speaking, however, it is unlikely that you would want to design an experiment with more than two or three IV’s. Smith/Davis (c) 2005 Prentice Hall

How many Groups or Levels? • Various factors are often designated by letters, so the first factor is labeled Factor A, the second as Factor B, and so on. • The levels within a factor are often designated by the letter that corresponds to the factor and a number to differentiate the different levels. • Thus, the two levels within the first factor would be labeled A1 and A2. Smith/Davis (c) 2005 Prentice Hall

How many Groups or Levels? • Main effect • A main effect refers to the sole effect of one IV in a factorial design. • Interaction • Another benefit that we get from doing an factorial experiment is the ability to examine potential interactions between the two IVs. • Significant interactions are found when the effects of one IV change as the level(s) of the other IV changes. In other words, the effects of one IV depend on the particular level of another IV. • A simple way to discern an interaction is to look at your findings graphically. If the lines on the graph are not parallel, then there likely is a significant interaction. Smith/Davis (c) 2005 Prentice Hall

Psychological Detective • Can you interpret the main effects in the figure below. Did customer hearing having an effect? Did the salesclerk sex have any effect? Study the graph to answer these questions. Smith/Davis (c) 2005 Prentice Hall

Assigning Participants to Groups • We have two options for this assignment – independent groups or correlated groups. • Factorial designs in which both IV’s involve random assignment may be called between-subjects factorial designs or completely randomized designs. • This decision is not as simple as in the two-group and multiple-group designs, each of which had only one IV. • All IV’s could have participants assigned randomly or in a correlated fashion, or we could have one IV with independent groups and one IV with correlated groups. This possibility is referred to as mixed assignment. Smith/Davis (c) 2005 Prentice Hall

Assigning Participants to Groups • Mixed assignment • A factorial design that has a mixture of independent groups for one IV and correlated groups for another IV. • In larger factorial designs, at least one IV has independent groups and at least one has correlated groups (also known as mixed groups). Smith/Davis (c) 2005 Prentice Hall

Nonrandom Assignment to Groups • In this section, we deal with factorial designs in which participant groups for all IV’s have been formed through nonrandom assignment. • We refer to such designs as completely within-groups (or within-subjects)designs. • We may want to resort to nonrandom assignment in order to assure the equality of participant groups before we conduct the experiment. Smith/Davis (c) 2005 Prentice Hall

Nonrandom Assignment to Groups • Matched Pairs or Sets • Matching can take place in either pairs or sets because factorial designs can use IV’s with two or more levels. • The more levels an IV has, the more work matching for that variable takes. • The more precise the match that is necessary, the more difficult matching becomes. Smith/Davis (c) 2005 Prentice Hall

Nonrandom Assignment to Groups • Repeated Measures • In a completely within-groups experiment using repeated measures, participants would take part fully and completely. • Participants take part in every possible treatment combination. • This requirement makes it difficult or impossible to conduct an experiment with repeated measures on multiple IV’s. • The smaller the design, the more feasible it is to include all participants in all conditions of the experiment. Smith/Davis (c) 2005 Prentice Hall

Nonrandom Assignment to Groups • Natural Pairs or Sets • Using natural groups in a totally within-subjects design has the same difficulties as the matched pairs or sets variation of this design, but it would be even harder. • The difficulty lies in being able to find an adequate number of naturally linked participants. Smith/Davis (c) 2005 Prentice Hall

Nonrandom Assignment to Groups • Mixed Assignment to Groups. • Mixed assignment designs involve a combination of random and nonrandom assignment, with at least one IV using each type of assignment to groups. • In a two-IV factorial design, mixed assignment involves one IV with random assignment and one IV with nonrandom assignment. • In such designs, the use of repeated measures is probably more likely than other types of nonrandom assignment. • Mixed designs combine the advantages of the two types of designs. • The conservation of participants through the use of repeated measures for a between-subjects variable makes for a popular and powerful design. Smith/Davis (c) 2005 Prentice Hall

Comparing the Factorial Design to Two-Group and Multiple-Group Designs • Two-group designs are ideal for a preliminary investigation of a particular IV in a presence-absence format. • The multiple-group design may be used to conduct more in-depth investigations of an IV that interests us. • We took the basic two-group design and extended it to include more levels of our IV. • We can make the same type of extension with factorial designs. • Just as with the multiple-group design, there is no limit to the number of levels for any IV in a factorial design. • The number of levels of the IV’s can be equal or unequal. • Interaction effects must be interpreted in factorial designs but not in two-group or multiple-group designs. • A good rule of thumb to follow is to choose the simplest research design that will adequately test your hypothesis Smith/Davis (c) 2005 Prentice Hall

Experimental Questions • Factorial designs provide considerable flexibility in devising an experiment to answer your questions. • The number of questions we can ask in a factorial experiment increases dramatically, but…. • When we ask additional questions, we must make certain that the questions coordinate with each other…experimental questions should not clash. • (e.g., it would not make sense to propose an experiment to examine the effects of self-esteem and eye color on test performance) Smith/Davis (c) 2005 Prentice Hall

Control Issues • We need to consider independent versus correlated groups in factorial designs. • A complicating factor for factorial designs is that we need to make this decision (independent vs. correlated groups) for each IV we include in an experiment. Smith/Davis (c) 2005 Prentice Hall

Practical Considerations • You are well advised to keep your experiment at the bare minimum necessary to answer the question(s) that most interest(s) you. • Bear in mind that you are complicating matters when you add IV’s and levels. • Remember the principle of parsimony mentioned in Chapter 10 and the KISS principle (Keep It Simple Stupid). Smith/Davis (c) 2005 Prentice Hall

Variations on Factorial Designs • Comparing Different Amounts of an IV • When you add a level to an IV in a factorial design, you add several groups to your experiment because each new level must be added under each level of your other independent variable(s). Smith/Davis (c) 2005 Prentice Hall

Comparing Different Amounts of an IV • When you add a level to an IV in a factorial design, you add several groups to your experiment because each new level must be added under each level of your other independent variable(s). • For example, expanding a 2 X 2 to a 3 X 2 design requires 6 groups rather than 4. • Adding levels in a factorial design increases groups in a multiplicative fashion. Smith/Davis (c) 2005 Prentice Hall

Using Measured IV’s • Ex post facto research • A research approach in which the experimenter cannot directly manipulate the IV but can only classify, categorize, or measure the IV because it is predetermined in the participants (e.g., IV = sex). Smith/Davis (c) 2005 Prentice Hall

Using Measured IV’s • Using a measured rather than a manipulated IV results in ex post facto research. • Without the control that comes from directly causing an IV to vary, we must exercise extreme caution in drawing conclusions from such studies. • We can develop an experiment that uses one manipulated IV and one measured IV at the same time. Smith/Davis (c) 2005 Prentice Hall

Dealing with More than Two IV’s • Designing an experiment with more than two IV’s is probably the most important variation of the factorial design. • The simplest possible factorial design with three IV’s (often referred to as a three-way design) has three IV’s, each with two levels. • This design represents a 2 X 2 X 2 experiment. • This design would require eight different groups if it is planned as a completely between-groups design. Smith/Davis (c) 2005 Prentice Hall

Chapter Fourteen