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Ch 8: Experimental Design Ch 9: Conducting Experiments.

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Presentation on theme: "Ch 8: Experimental Design Ch 9: Conducting Experiments."— Presentation transcript:

1 Ch 8: Experimental Design Ch 9: Conducting Experiments

2 Uses of Experimental Research Test hypotheses derived from theories Test the effectiveness of a treatment or program Examine the causes of behavior

3 Conducting Experimental Research  Manipulate independent variable to see effect on dependent variable  Compare groups in terms of their scores on the dependent variable  All other variables kept constant through direct experimental control and/or randomization

4 Independent Variable This variable is manipulated (controlled) by the experimenter and has at least two different levels (conditions) 1000 mg 0 mg=placebo

5 Manipulating the Independent Variable  Straightforward manipulations  Amount of substance administered  Written instructions  Verbal material  Visual material  Staged manipulations  Often employ confederates  Events are staged or manipulated to:  Create some psychological state  Simulate some situation that occurs in the real world

6 Dependent Variable This variable is measured by the experimenter and is used to determine the effect of the independent variable no pain mild moderate severe

7 Measuring the Dependent Variable  Types of measures Self-report measures Behavioral measures Physiological measures

8 Sensitivity of the Dependent Variable  The independent variable can appear to have no effect on the dependent variable when there is a Ceiling effect—participants quickly reach the maximum performance level Floor effect—a task is so difficult that hardly anyone can perform well

9 Posttest-only Design  Obtain two equivalent groups of participants (R=random assignment)  Introduce the independent variable  Measure the effect of the independent variable on the dependent variable

10 Posttest-only Design Involves random assignment of individuals to the different conditions of the independent variable

11 Random assignment A method for placing subjects in conditions prior to implementing the independent variable Every individual has the same chance of being placed in a given condition

12 Pretest-posttest design  Same as a posttest-only design but adds a pretest before the experimental manipulation  Allows the researcher to ascertain if the groups are equivalent at the beginning of the experiment  Example: Are kids healthier on some dimension (weight, stamina) after going through an athletics program?

13 1. Assess equivalency with small sample size 2. Assess mortality (attrition or dropout factor) Advantages of the pretest-posttest design

14 Mortality Mechanical subject loss: equipment failure or experimenter error leads to loss Selective subject loss: some characteristic of participant is responsible for loss

15 Disadvantages of the pretest-posttest design Time consuming Awkward to administer Sensitizes participants to what is being studied Demand characteristics: Cues and information a participant uses to guide their behavior in a psychological study “Is research on cultural acceptance?”

16 Demand characteristics oPossible solutions: disguise pretest embed the pretest in another measure (filler questions) concealed observation Placebo control group —used to assess whether participants’ expectancies contribute to the outcome of an experiment

17 Internal validity Occurs when we are able to confidently state that the independent variable caused the differences we observe Causal inferences can be made when internal validity is present

18 Confounding This occurs when the variable of interest and a different potential independent variable are allowed to covary Represents an alternative explanation for a study’s findings Threatens internal validity

19 Other Threats to Internal Validity Intact groups Extraneous variables Experimenter effects

20 Intact groups This occurs when groups are formed prior to the start of an experiment Selection differences: systematic ways in which people can differ

21 Selection differences Characteristics of people that differ or vary: Physical characteristics: sex, race Social characteristics: ethnicity, religion, marital status Personality characteristics: extraversion, emotional stability Mental health characteristics: depression, anxiety

22 Extraneous variables Other practical considerations that may confound an experiment

23 Experimenter effects Biases that occur when experimenters’ expectancies regarding the outcome of the experiment influence their behavior toward participants in different conditions Control by automating procedures as much as possible

24 Double-blind experiment A procedure in which both the participants and the experimenters are unaware of which condition is being administered Controls for both demand characteristics and experimenter effects

25 Experimental control techniques Methods for ruling out other possible causes for the outcome

26 Assigning Participants to Experimental Conditions  Independent groups design Participants randomly assigned to conditions Participants are in only one group Low- meaningful High- meaningful 15 randomly assigned participants Another 15 randomly assigned participants Meaningfulness

27 Assigning Participants to Experimental Conditions  Repeated measures design The same participants are in all of the groups Low- meaningful High- meaningful 15 participantsThe SAME 15 participants Meaningfulness

28 Repeated Measures Design  Advantages  Fewer participants  Extremely sensitive to statistical differences (more likely to detect an effect of the IV on the DV)  Disadvantages  Order effects  Practice effects  Fatigue effects  Contrast effects

29 Minimizing order effects  Counterbalancing 1.Complete counterbalancing—all possible orders of presentation are included in the experiment

30 Matched pairs design Ensures groups are equivalent on the matching variable prior to the IV Match participants on a particular characteristic After matching, randomly assign to experimental conditions


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