Chapter Nine: Designing, Conducting, Analyzing, and Interpreting Experiments with Two Groups

1. Chapter Nine:Designing, Conducting, Analyzing, and Interpreting Experiments with Two Groups

2. Experimental Design: The Basic Building Blocks • Experimental design

3. Experimental Design: The Basic Building Blocks • Experimental design • The general plan for selecting participants, assigning participants to experimental conditions, controlling extraneous variables, and gathering data.

4. The Two-Group Design • Principle of parsimony

5. 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razor) • The belief that explanations of phenomena and events should remain simple until the simple explanations are no longer valid.

6. The Two-Group Design • Independent variable

7. 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.

8. 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 9 and 10 deal with research designs that have one IV. • Chapter 11 deals with research designs that have more than one IV. • Dependent variable (DV)

9. 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. • 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.

10. The Two-Group Design • How many groups?

11. The Two-Group Design • How many groups? • Although an experiment can have only one IV, it must have at least two groups.

12. 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.

13. 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.

14. The Two-Group Design • Extraneous variables

15. The Two-Group Design • Extraneous variables • Undesired variables that may operate to influence the DV and, thus, invalidate an experiment.

16. The Two-Group Design • Levels

17. 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.

18. 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 iscontrasted with the absence of the IV.

19. 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.

20. The Two-Group Design • Experimental group

21. The Two-Group Design • Experimental group • In a two-group design, the group of participants that receives the IV.

22. The Two-Group Design • Experimental group • In a two-group design, the group of participants that receives the IV. • Control group

23. The Two-Group Design • Experimental 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.

24. The Two-Group Design • Assigning Participants to Groups

25. The Two-Group Design • Assigning Participants to Groups • Random Assignment

26. The Two-Group Design • Assigning Participants to Groups • Random Assignment • A method of assigning research participants to groups so that each participants to groups so that each participant has an equal chance of being in any group.

27. The Two-Group Design • Assigning Participants to Groups • Random Assignment • A method of assigning research participants to groups so that each 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.

28. The Two-Group Design • Assigning Participants to Groups • Random Assignment • A method of assigning research participants to groups so that each 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.

29. The Two-Group Design • Assigning Participants to Groups • Random Assignment • A method of assigning research participants to groups so that each 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.

30. The Two-Group Design • Independent groups

31. The Two-Group Design • Independent groups • The participants in one group have absolutely no ties or links to the participants in the other group.

32. The Two-Group Design • Independent groups • The participants in one group have absolutely no ties or links to the participants in the other group. • Between-subjects comparison

33. The Two-Group Design • Independent groups • 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.

34. The Two-Group Design • Confounded experiment

35. The Two-Group Design • Confounded experiment • An experiment in which an extraneous variable varies systematically with the IV.

36. 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.

37. 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.

38. The Two-Group Design • Nonrandom Assignment to Groups.

39. 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.

40. 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.

41. 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?

42. The Two-Group Design • Correlated assignment

43. 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.

44. 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 than randomly assigned to treatment conditions (also known as paired or matched assignment).

45. The Two-Group Design • Correlated assignment • Matched pairs

46. 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.

47. 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.

48. 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.

49. 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.

50. The Two-Group Design • Correlated assignment • Matched pairs • Repeated measures