1. Chapter Ten: Designing, Conducting, Analyzing, and Interpreting Experiments with Two Groups
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

2. Experimental Design: The Basic Building Blocks
The general plan for selecting participants, assigning participants to experimental conditions, controlling extraneous variables, and gathering data.
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3. The Two-Group Design Principle of Parsimony (Occam’s Razor)
The belief that explanations of phenomena and events should remain simple until the simple explanations are no longer valid.
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4. The Two-Group Design Independent Variable (IV)
A stimulus or aspect of the environment that the experimenter directly manipulates to determine its influences on behavior.
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5. The Two-Group Design Independent Variable (IV)
A stimulus or aspect of the environment that the experimenter directly manipulated to determine its influences on behavior.
Chapters 10 and 11 deal with research designs that have one IV.
Chapter 12 deals with research designs that have more than one IV.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

6. The Two-Group Design Independent Variable (IV) Dependent Variable (DV)
A stimulus or aspect of the environment that the experimenter directly manipulated to determine its influences on behavior.
Dependent Variable (DV)
A response or behavior that is measured. It is desired that changes in the DV be directly related to manipulation of the IV.
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7. The Two-Group Design How Many Groups?
Although an experiment can have only one IV, it must have at least two groups.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

8. The Two-Group Design How Many Groups?
Although an experiment can have only one IV, it must have at least two groups.
The simplest way to find out whether our IV caused a change in behavior is to compare some research participants who have received our IV to some others who have not received the IV.
If those two groups differ, and we are assured that we controlled potential extraneous variables, then we conclude that the IV caused the participants to differ.
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9. The Two-Group Design Extraneous Variables
Undesired variables that may operate to influence the DV and, thus, invalidate an experiment.
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10. The Two-Group Design Levels
The most common manner of creating two groups with one IV is to present some amount or type of IV to one group and to withhold that IV from the second group.
Thus, the presence of the IV is contrasted with the absence of the IV.
These differing levels of the IV are referred to as the levels (also known as treatment conditions) of the IV.
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11. The Two-Group Design Experimental Group
In a two-group design, the group of participants that receives the IV.
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12. The Two-Group Design Experimental Group Control Group
In a two-group design, the group of participants that receives the IV.
Control Group
In a two-group design, the group of participants that does not receive the IV.
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13. The Two-Group Design Assigning Participants to Groups
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14. The Two-Group Design Assigning Participants to Groups
Random Assignment
A method of assigning research participants to groups so that each participant has an equal chance of being in any group.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

15. The Two-Group Design Assigning Participants to Groups
Random Assignment
A method of assigning research participants to groups so that each participant has an equal chance of being in any group.
Random assignment is not the same as random selection.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

16. The Two-Group Design Assigning Participants to Groups
Random Assignment
A method of assigning research participants to groups so that each participant has an equal chance of being in any group.
Random assignment is not the same as random selection.
When we randomly assign participants to groups, we have created what are known as independent groups.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

17. The Two-Group Design Assigning Participants to Groups
Random Assignment
A method of assigning research participants to groups so that each participant has an equal chance of being in any group.
Random assignment is not the same as random selection.
When we randomly assign participants to groups, we have created what are known as independent groups.
When we wish to compare the performance of participants in these two groups, we are making what is known as a between-subjects comparison.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

18. The Two-Group Design Independent Groups
The participants in one group have absolutely no ties or links to the participants in the other group.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

19. The Two-Group Design Independent Groups Between-Subjects Comparison
The participants in one group have absolutely no ties or links to the participants in the other group.
Between-Subjects Comparison
Refers to a contrast between groups of participants who were randomly assigned to groups.
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20. The Two-Group Design Confounded Experiment
An experiment in which an extraneous variable varies systematically with the IV.
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21. The Two-Group Design Confounded Experiment
An experiment in which an extraneous variable varies systematically with the IV.
Confounding makes drawing a cause-and-effect relation impossible.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

22. The Two-Group Design Confounded Experiment
An experiment in which an extraneous variable varies systematically with the IV.
Confounding makes drawing a cause-and-effect relation impossible.
Confounding may occur if participants are not equal before the start of the experiment.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

23. The Two-Group Design Nonrandom Assignment to Groups
Random assignment tends to create equal groups in the long run. As groups get larger, we can place more confidence in random assignment achieving what we want it to.
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24. The Two-Group Design Nonrandom Assignment to Groups
Random assignment tends to create equal groups in the long run.
As groups get larger, we can place more confidence in random assignment achieving what we want it to.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

25. The Two-Group Design Nonrandom Assignment to Groups
Random assignment tends to create equal groups in the long run.
As groups get larger, we can place more confidence in random assignment achieving what we want it to.
If we are faced with a situation in which we have few potential research participants and we are worried that random assignment may not create equal groups, what can we do?
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26. The Two-Group Design Correlated Assignment
A method of assigning research participants to groups so that there is a relationship between small numbers of participants.
These small groups are then randomly assigned to treatment conditions (also known as paired or matched assignment).
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27. The Two-Group Design Correlated Assignment Matched pairs
Research participants in a two-group design who are measured and equated on some variable before the experiment.
Typically we measure a variable that could result in confounding if not controlled.
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28. The Two-Group Design Correlated Assignment Matched pairs
Research participants in a two-group design who are measured and equated on some variable before the experiment.
Typically we measure a variable that could result in confounding if not controlled.
After we have measured this variable, we create pairs of participants that are equal on this variable.
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29. The Two-Group Design Correlated Assignment Matched pairs
Research participants in a two-group design who are measured and equated on some variable before the experiment.
Typically we measure a variable that could result in confounding if not controlled.
After we have measured this variable, we create pairs of participants that are equal on this variable.
After we have created our matched pairs, we then randomly assign participants from these pairs to the different treatment conditions.
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30. The Two-Group Design Correlated Assignment Matched pairs
Repeated measures
The same participants are tested in both treatment conditions of our experiment.
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31. The Two-Group Design Correlated Assignment Matched pairs
Repeated measures
The same participants are tested in both treatment conditions of our experiment.
The matched pairs are perfectly equal because they consist of the same people or animals tested across the entire experiment.
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32. The Two-Group Design Correlated Assignment Matched pairs
Repeated measures
The same participants are tested in both treatment conditions of our experiment.
The matched pairs are perfectly equal because they consist of the same people or animals tested across the entire experiment.
No extraneous variables should be able to confound this situation because any difference between the participants’ performance in the two treatment conditions is due to the IV.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

33. The Two-Group Design Correlated Assignment Matched pairs
Repeated measures
The same participants are tested in both treatment conditions of our experiment.
The matched pairs are perfectly equal because they consist of the same people or animals tested across the entire experiment.
No extraneous variables should be able to confound this situation because any difference between the participants’ performance in the two treatment conditions is due to the IV.
In this type of experiment, participants serve as their own controls.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

34. The Two-Group Design Correlated Assignment Matched pairs
Repeated measures
Natural pairs
Pairs of participants are created from naturally occurring pairs (e.g. biologically or socially related).
For example, psychologists who study intelligence often use twins as their research participants.
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35. The Two-Group Design Within-Subjects Comparison
Refers to a contrast between groups of participants who were assigned to groups through matched pairs, natural pairs, or repeated measures.
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36. The Two-Group Design Within-Subjects Comparison
Refers to a contrast between groups of participants who were assigned to groups through matched pairs, natural pairs, or repeated measures.
We are essentially comparing scores within the same participants (subjects).
Although this direct comparison is literally true only for repeated-measures designs, participants in matched or natural pairs are the same with regard to the matching variable.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

37. Comparing Two-Group Designs
Choosing a Two-Group Design
Random assignment should should equate your groups adequately (assuming that you have large groups).
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

38. Comparing Two-Group Designs
Choosing a Two-Group Design
Random assignment should should equate your groups adequately (assuming that you have large groups).
If you are using 20 or more participants per group, you can feel fairly safe that randomization will create equal groups.
If you are using 5 or fewer participants in a group, randomization may not work.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

39. Comparing Two-Group Designs
Choosing a two-group design
Advantages of correlated groups designs
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

40. Comparing Two-Group Designs
Choosing a two-group design
Advantages of correlated groups designs
Control issues
The three methods for creating correlated-groups designs give us greater certainty of group equality.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

41. Comparing Two-Group Designs
Choosing a Two-Group Design
Advantages of Correlated Groups Designs
Control issues
Statistical issues
Correlated-groups designs can help reduce error variation.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

42. Comparing Two-Group Designs
Error Variability
Variability in DV scores that is due to factors other than the IV – individual differences, measurement error, and extraneous variation (also known as within-groups variability).
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

43. Comparing Two-Group Designs
Choosing a Two-Group Design
Advantages of Correlated Groups Designs
Advantages of Independent-Groups Designs
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44. Comparing Two-Group Designs
Choosing a Two-Group Design
Advantages of Correlated Groups Designs
Advantages of Independent-Groups Designs
Simplicity
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

45. Comparing Two-Group Designs
Choosing a Two-Group Design
Advantages of Correlated Groups Designs
Advantages of Independent-Groups Designs
Simplicity
Use of correlated-groups designs is impossible in some situations.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

46. Comparing Two-Group Designs
True Experiment
An experiment in which the experimenter directly manipulates the IV.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

47. Comparing Two-Group Designs
True Experiment
An experiment in which the experimenter directly manipulates the IV.
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

48. Statistical Analysis: What Do Your Data Show?
The Relation Between Experimental Design and Statistics
Selecting the appropriate experimental design determines the particular statistical test you will use to analyze your data.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

49. Statistical Analysis: What Do Your Data Show?
The Relation Between Experimental Design and Statistics
Selecting the appropriate experimental design determines the particular statistical test you will use to analyze your data.
You should determine your experimental design before you begin collecting data to ensure there will be an appropriate statistical test you can use to analyze your data.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

50. Statistical Analysis: What Do Your Data Show?
Analyzing Two-Group Designs
For a two-independent groups design, use a t test for independent samples to analyze your data.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

51. Statistical Analysis: What Do Your Data Show?
Analyzing Two-Group Designs
For a two-independent groups design, use a t test for independent samples to analyze your data.
For a two-correlated-groups design, analyze your data with a t test for correlated samples (a.k.a dependent t test, a within-groups t test, or a paired t test).
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

52. Statistical Analysis: What Do Your Data Show?
Interpretation: Making Sense of Your Statistics
Statistics are a tool to help you understand the data from your experiment.
Statistics are useless if you don’t know how to interpret them.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

53. Statistical Analysis: What Do Your Data Show?
Interpreting Computer Statistical Output
The t test for independent samples
Homogeneity of variance
The assumption that the variances are equal for two (or more) groups you plan to compare statistically.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

54. Statistical Analysis: What Do Your Data Show?
Interpreting Computer Statistical Output
The t test for independent samples
Homogeneity of variance
The assumption that the variances are equal for two (or more) groups you plan to compare statistically.
Heterogeneity of variance
Occurs when we do not have homogeneity of variance.
This means that our two (or more) groups’ variances are not equivalent.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

55. Statistical Analysis: What Do Your Data Show?
Interpreting Computer Statistical Output
The t test for independent samples
Generally speaking, t tests are robust with regard to the assumption of homogeneity (Kirk, 1968).
A robust test is one that can tolerate violations of its assumptions and still provide accurate answers.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

56. Statistical Analysis: What Do Your Data Show?
Interpreting Computer Statistical Output
The t test for independent samples
Translating statistics into words
If two equal groups began the experiment and they are now unequal, to what can we attribute the difference?
If our controls have been adequate, our only choice is to assume that the difference between the groups is due to the IV.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

57. Statistical Analysis: What Do Your Data Show?
Interpreting Computer Statistical Output
The t test for independent samples
Translating statistics into words
For example, if you were writing an interpretation of the results from the sample experiment in your text, you might write something like the following:
Salesclerks who waited on well-dressed customers (M = 43.38, SD = 10.11) took significantly less time, t(14) = 2.61, p = .021, to respond to customers than salespeople who waited on customers dressed in sloppy clothing (M = 63.25, SD = 12.54). The effect size, estimated with Cohen’s d, was .92.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

58. Statistical Analysis: What Do Your Data Show?
Interpreting Computer Statistical Output
The t test for correlated samples
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

59. Statistical Analysis: What Do Your Data Show?
Translating Statistics Into Words
Here is the example from your text:
Salespeople who waited on well-dressed customers (M = 48.38, SD = 10.11) took significantly less time, t(7) = 5.47, p = .001, to respond to the customers than when they waited on customers dressed in sloppy clothes (M = 63.25, SD = 12.54). The effect size, estimated with Cohen’s d, was 1.93.
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60. The Continuing Research Problem
It is rare for a psychologist to conduct a single research project and stop at that point because that one research project had answered all the questions about the particular topic.
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61. The Continuing Research Problem
Let’s review the steps involved in choosing a research design (using the hypothetical example from the text).
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62. The Continuing Research Problem
After reviewing relevant research literature, we chose our IV (style of dress) and our DV (salesclerk response time).
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63. The Continuing Research Problem
After reviewing relevant research literature, we chose our IV (style of dress) and our DV (salesclerk response time).
Because we were conducting a preliminary investigation into the effects of clothing on salesclerks’ reactions, we decided to test only one IV (the style of dress).
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

64. The Continuing Research Problem
After reviewing relevant research literature, we chose our IV (style of dress) and our DV (salesclerk response time).
Because we were conducting a preliminary investigation into the effects of clothing on salesclerks’ reactions, we decided to test only one IV (the style of dress).
Because we wanted to determine only whether clothing style can affect the performance of salespeople, we chose to use only two levels of the IV (dressy clothing vs. sloppy clothing).
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

65. The Continuing Research Problem
After reviewing relevant research literature, we chose our IV (style of dress) and our DV (salesclerk response time).
Because we were conducting a preliminary investigation into the effects of clothing on salesclerks’ reactions, we decided to test only one IV (the style of dress).
Because we wanted to determine only whether clothing style can affect the performance of salespeople, we chose to use only two levels of the IV (dressy clothing vs. sloppy clothing).
If we have a large number of participants available, then we can use random assignment, which yields independent groups. In this case, we would use the two-independent-groups design and analyze the data with a t test for independent groups.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

66. The Continuing Research Problem
Because we wanted to determine only whether clothing style can affect the performance of salespeople, we chose to use only two levels of the IV (dressy clothing vs. sloppy clothing).
If we have a large number of participants available, then we can use random assignment, which yields independent groups. In this case, we would use the two-independent-groups design and analyze the data with a t test for independent groups.
If we expect to have a small number of participants and need to exert the maximum degree of control, we choose to use a design with repeated measures or matched groups, thus resulting in correlated groups. Therefore, we would use a two-correlated-groups design for the experiment and analyze the data with a t test for correlated groups.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

67. The Continuing Research Problem
If we have a large number of participants available, then we can use random assignment, which yields independent groups. In this case, we would use the two-independent-groups design and analyze the data with a t test for independent groups.
If we expect to have a small number of participants and need to exert the maximum degree of control, we choose to use a design with repeated measures or matched groups, thus resulting in correlated groups. Therefore, we would use a two-correlated-groups design for the experiment and analyze the data with a t test for correlated groups.
We concluded that salespeople responded more quickly to customers in dressy clothes than customers dressed in sloppy clothes.
The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education