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Chapter 8


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Anna Nagy

Chapter 8 Confounding and Internal Validity - IV is manipulated to create groups that differ in levels of variable and then compares the groups in terms of DV - Other variables are kept constant = through randomization or direct experimental control o If scores of groups are different = IV caused the results o Because the only difference b/w groups is manipulated variable - confounding variable = variable that varies along with the IV o confounding occurs when effects of IV and an uncontrolled variable are intertwined so you cant determine which of the variables is responsible for observed effect. - good experimental design involves eliminating possible confounding that results in alternative explanations - internal validity = when the results of an experiment can confidently be attributed to the effect of IV o to achieve this: to design and conduct the experiment so that only IV can be the cause of results Basic experiments 2 variables: 1. Independent variable (IV) – two levels: a. Experimental group b. Control group 2. Dependent Variable (DV)  only difference between two groups is manipulated variable  experimental method involves control over extraneous variables, through keeping variables constant (experimental control) or using randomization to make sure that any extraneous variables will affect both groups equally 2 forms of basic, simple experimental design 1. posttest-only design - obtain two equivalent groups of participants - introduce the IV - measure the effect of IV on DV - REFER TO PAGE 149 FOR SUMMARY - Steps: o 1. Choose the participants and assign them to two groups  procedures must achieve equivalent groups to eliminate any potential selection differences (people selected to be in conditions can’t differ in any systematic way) 1  has to be randomly assigned to two groups/conditions or  by having the same participants participate in both conditions. o 2. Choose two levels of IV  such as an experimental group that receives a treatment and a control group does not  using two different amounts of IV o 3. effect of IV is measured  same measurement procedure is used for both groups so that comparison of two groups is possible  no confounding variables = any diff b/w groups on DV MUST be attributed to effect of IV  high degree of internal validity! 2. Pretest-Posttest Design - pretest is given before the experimental manipulation is introduced - possible to establish that the groups were equivalent at the beginning of experiment - not necessary if participants have been randomly assigned to the two groups - with sufficiently large sample of participants = random assignment will produce groups that are virtually identical in all respects o larger the sample = less likelihood there is that the groups will differ in any systematic way PRIOR to manipulation of IV o increasing likelihood that any difference b/w groups on DV is due to the effect of IV o minimum of 20 to 30 participants per condition PRETEST-POSSTEST DESIGN Advantages Disadvantages (of pretest) (of pretest) Enables to assess whether the groups Time consuming & awkward to administer were equivalent to begin with Can measure the extent of change in Sensitize participants to what you are each individual studying * Mortality (dropout factor) * May not have an effect in real world = reduces external validity Can use to select the participants for experiment * Mortality (dropout factor) = diff mortality can make the groups nonequivalent = e.g. smoking example  when posttest is given only the light smokers would remain after heavy smokers left, comparison of experimental and control groups would show less smoking in the experimental group even if the program had no effect = mortality becomes an alternative explanation for the results 2 = pretest enables you to assess the effects of mortality & whether mortality is a plausible alternative explanation. - you can look at the pretest scores of the dropouts and know whether mortality affected the final results Sensitize participants to what you are studying * = enabling them to figure out your hypothesis = they may react differently to manipulation = IV may not have an effect in real world = reduces external validity  assess the impact of pretest directly with a combination of both posttest-only and pretest-posttest design.  Half the participants receive only the posttest  Other half receive both the pretest and posttest  Solomon four-group design  If there is no impact of pretest = posttest scores will be the same in two control groups (with and without the pretest) and in the 2 experimental groups  Outcome in which there is a difference between the treatment and control groups = when there is a pretest  There is no group difference = when the pretest is absent! Assigning Participants to Experimental conditions 3 ways: I. independent groups design - participants are randomly assigned to various conditions so that each participates in only one group - random assignment o will prevent any systematic biases o groups will be equivalent in terms of participant characteristics such as income, age, intelligence, etc. II. repeated measures - same participants are in all conditions - each participant is measured after receiving each level of IV - participants are repeatedly measured on the DV after being in each condition of experiment o a. advantages of repeated measures  fewer research participants are needed  less expensive  extremely sensitive to finding statistically significant differences between groups  because we have data from the same people in both conditions 3  much easier to separate the systematic individual differences from effect of IV  more likely to detect an effect of IV on DV o b. disadvantages of repeated measures o different conditions must be presented in a particular sequence o order effect – order of presenting the treatments affects the DV - greater recall in high-meaningful condition could be attributed to the fact that the high-meaningful task came second in the order of presentation of conditions  types of order effect:  1. Practice effect – improvement in performance as a
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