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Chapter 8 PsyB01.doc

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University of Toronto Scarborough
Connie Boudens

Chapter 8 – Experimental Design - In the experimental method, all extraneous variables are controlled Confounding and Internal Validity - The experimental method has the advantage of allowing a relatively unambiguous interpretation of results: o The researcher manipulates the independent variable to create groups that differ in the levels of the variable and then compares the groups in terms of their scores on the dependent variable o All other variables are kept constant, either through direct experimental control or through randomization o If the scores of the groups are different, the researcher can concluded that the independent variable caused the results because the only difference between the groups is the manipulated variable - A confounding variable is a variable that varies along with the independent variable; confounding occurs when the effects of the independent variable and an uncontrolled variable are intertwined so you cannot determine which of the variables is responsible for the observed effect - When the results of an experiment can confidently be attributed to the effect of the independent variable, the experimenter is said to have internal validity Basic Experiments - The experimental method involves control over extraneous variables, through either keeping such variables constant (experimental control) or using randomization to make sure that any extraneous variables will affect both groups equally Posttest-Only Design - A researcher using a posttest-only design must obtain two equivalent group of participants, introduce the independent variable, and measure the effect of the independent variable on the dependent variable - Selection differences: the people selected to be in the conditions cannot differ in any systematic way o The groups can be made equivalent by randomly assigning participants to the two conditions or by having the same participants participate in both conditions - Next, the researcher must choose two levels of the independent variable, such as an experimental group that receives a treatment and a control group that does not o Another approach would be to use two different amounts of the independent variable - Finally, the effect of the independent variable is measured; the same measurement procedure is used for both groups, so that comparison of the two groups is possible - Because the groups were equivalent to begin with and there were no confounding variables, any difference between the groups on the dependent variable must be attributed to the effect of the independent variable Pretest-Posttest Design - The only difference between the posttest-only design and the pretest-posttest design is that in the latter a pretest is given before the experimental manipulation is introduced - This design makes it possible to ascertain that the groups were equivalent at the beginning of the experiment - The larger the sample, the less likelihood there is that the groups will differ in any systematic way prior to the manipulation of the independent variable o At the same time, there is an increasing likelihood that any difference between the groups on the dependent variable is due to the effect of the independent variable Advantages and Disadvantages of the Two Designs - Although randomization is likely to produce equivalent groups, it is possible that, with small sample sizes, the groups will not be equal; thus, a pretest enables the researchers to assess whether the groups were in fact equivalent to begin with - Sometimes a pretest is necessary to select the participants in the experiment - The researcher who uses a pretest can measure the extent of change in each individual - A pretest is also necessary whenever there is a possibility that participants will drop out of the experiment; this is most likely to occur in a study that lasts over a long period of time - The dropout factor in experiments is called mortality - Use of a pretest enables you to assess the effects of mortality; you can look at the pretest scores of the dropouts and know whether mortality affected the final results - One disadvantage of a pretest is that it may be time-consuming and awkward to administer in the context of the particular experimental procedures used - A pretest can sensitize participants to what you are studying, enabling them to figure out your hypothesis; they may then react differently to the manipulation than they would have without the pretest - When a pretest affects the way participants react to the manipulation, it is very difficult to generalize the results to people who have not received a pretest - If awareness of the pretest is a problem, the pretest can be disguised: o One way to do this is by administering it in a completely different situation with a different experimenter o Another approach is to embed the pretest in a set of irrelevant measures so it is not obvious that the researcher is interested in a particular topic - It is also possible to assess the impact of the pretest directly with a combination of both the posttest-only and the pretest-posttest design - A Solomon four-group design is when half the participants receive only the posttest, and the other half receive both the pretest and the posttest o If there is no impact of the pretest, the posttest scores will be the same in the two control groups (with and without the pretest) and in the two experimental groups Assigning Participants to Experimental Conditions - There are two basic ways of assigning participants to experimental conditions: o An independent groups design is when participants are randomly assigned to the various conditions so that each participates in only one group o A repeated measures design is when each participant is assigned to both levels of the independent variable and measured afterwards Independent Groups Design - In an independent groups design, different participants are assigned to each of the conditions using random assignment; this means that the decision to assign an individual to a particular condition is completely random and beyond the control of the researcher - Random assignment will prevent any systematic biases, and the groups will be equivalent in terms of participant characteristics such as income, intelligence, age, or political attitudes o In this way, participant differences cannot be an explanation for results of the experiment Repeated Measures Design - In a repeated measures design, the same individuals will participate in both conditions; participants are repeatedly measured on the dependent variable after being in each condition of the experiment Advantages and Disadvantages of Repeated Measures Design - An obvious advantage is that fewer research participants are needed because each individual participates in all conditions - An additional advantage of repeated measures designs is that
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