The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Chapter Twelve: Designing, Conducting, Analyzing, and Interpreting Experiments with Multiple Independent Variables Chapter Twelve: Designing, Conducting, Analyzing, and Interpreting Experiments with Multiple Independent Variables

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Hypothetical Experiment Suppose you were interested in determining whether Men or Women were more likely to convict a courtroom defendant. You design an ex post facto study with randomly selected groups of men and women. You present them with a defendant who tells a story, and ask them to rate the likelihood that the defendant was guilty. What other factor might influence these results?

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education 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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Experimental Design: Doubling the Basic Building Block Factors Synonymous with IVs

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education 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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Experimental Design: Doubling the Basic Building Block Factorial designs look at combinations of IVs at the same time

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education How Many IVs? The factorial design gets its name because we refer to each IV as a factor. Multiple IVs yield a factorial design. Theoretically, there is no limit to the number of IVs 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 IVs.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education How many Groups or Levels? The simplest possible factorial design is known as a 2 X 2 design.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education How many Groups or Levels? The number of numbers tells us how many IVs there are. The value of each number tells us how many levels each IV has. 2 x 2 (say 2 by 2) means we have two independent variables with two levels each. What would you call an experiment that had three levels of a drug (0, low, and high) given to ADD-diagnose and non- diagnosed children?

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education 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 Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education 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 (A sub 1) and A2 (A sub 2).

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Assigning Participants to Groups We have two options for this assignment – independent groups or correlated groups. Independent – participants are randomly assigned Correlated Matched sets Natural sets Repeated measures

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Assigning Participants to Groups Independent – Each participant is tested in only ONE condition of the study – In a two x two design, you would need separate participants for all four groups.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education 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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education 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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Assigning Participants to Groups Mixed designs Combine independent and repeated measures: Hypothetical experiment: A researcher wants to know whether short or long words are remembered more easily and he wants to know whether the delay between testing and recall effects the words differently. He gives participants a list containing BOTH short and long words. He then randomly assigns participants to have either a short or a long delay.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education What does a factorial design tell us? Main Effect A main effect refers to the sole effect of one IV in a factorial design. Interaction Does the level of one variable influence the effect of the other variable?

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Hypothetical Problem Example #1 : A researcher wishes to examine whether time of day (morning versus afternoon) affects the memory of older and younger participants. He obtains a sample of 40 older individuals and 40 younger individuals and gives them a passage to read. They are later given a test over the passage (scores on the test could range from 0 to 30). Half of the individuals are randomly assigned to read the passage and take the test in the morning, and the other half do so in the afternoon. The data are reported in the table below (the numbers represent mean test scores for each group).

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Data from problem 1 Morning TestingAfternoon Testing Older AdultsM = 15M = 8 Younger AdultsM = 16M = 20

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Hypothetical Problem Example #2 : A researcher takes a sample of 40 introverts and 40 extroverts and asks them to solve problems in either a crowded room or an uncrowded room. The researcher measures the number of problems solved (numbers can range from 0 problems solved to 25 problems solved).

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Data from problem 2 Crowded RoomUncrowded Room IntrovertsM = 12 ExtrovertsM = 18

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Hypothetical Problem Example #3 : A researcher wishes to examine whether people high or low in self-esteem react differently to different kinds of therapy for depression. She takes a sample of 120 people (half have high self-esteem and half have low self-esteem) and randomly assigns them to one of three conditions (humanistic therapy, cognitive- behavioral therapy, and a control group of no therapy). She records their level of depression after 3 months of therapy. Scores can range from 1 (very little depression ) to 30 ( extreme depression ).

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Data from problem 3:In the boxes below, write in a pattern of means that would reveal a main effect of type of therapy. HumanisticCognitive- Behavioral Control Group High Self- Esteem M = Low Self- Esteem M =

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Data from problem 3:In the boxes below, write in a pattern of means that would reveal a main effect of self esteem. HumanisticCognitive- Behavioral Control Group High Self- Esteem M = Low Self- Esteem M =

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Expanding the factorial design 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 six groups rather than four. Adding levels in a factorial design increases groups in a multiplicative fashion.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Using Measured IVs Using a measured rather than a manipulated IV results in 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).

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Dealing with More than Two IVs Designing an experiment with more than two IVs is probably the most important variation of the factorial design.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Dealing with More than Two IVs The simplest possible factorial design with three IVs (often referred to as a three-way design ) has three IVs, 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.