PSY3213C Exam 3 Study Guide Chapter 10 Introduction to Simple Experiments What is a matched group design Matched group design instead of participants being randomly assigned the experimental groups are matched and are equal on characteristics that might influence the dependent variable o Example Study on if video games reverse cognitive declines associated with aging Age has a large effect on academic performance so it needs to be controlled for To do this different experimental groups must consist of individuals close in age What is the difference between within subject design and between subject design What are the advantages disadvantages of each Within Subjects Design the same participants are exposed to every group related to the variable One group of participants o Concurrent Measures Design participants are exposed to all the levels of an independent variable at roughly the same time and a single attitudinal or behavioral preference is the dependent measure Example Harlow s study with baby rhesus monkeys o Repeated Measures Design participants are measured on a dependent variable more than once that is after exposure to each level of the independent variable Bick and Dozier 2008 One group would interact with their own toddler Then oxytocin would be measured This would be done again with a new toddler Carryover Effects The problem with this is that a baseline oxytocin measurement is not taken And the oxytocin from the first toddler can carry over into the next trial A way to fix this might be to mix up the order of which child they interact with first through random assignment The problem now is that you need twice as many participants o Advantages Disadvantages of Within Subject Designs Advantages It reduces error variance due to individual differences among subjects across treatment groups There are reduced error variance results in a more powerful design The effects of the independent variable are more likely to be detected It requires fewer participants There are more demands put on participants longer study more complicated study may increase drop out rate When participants do drop out all their data is unusable Mistakes more costly again because you lose a large amount of Disadvantages data Carryover Effects Between Subjects Design different groups of participants are exposed to the different levels of the variable Advantages Disadvantages can be asserted from the advantages disadvantages listed above o Posttest Only Design participants are randomly assigned to independent variable groups and are tested on the dependent variable once We measure and record the dependent variable after treatment and use statistics to determine whether the independent variable had a statistically reliable effect Advantages It is simple Relatively few participants are needed the data and stats are easy and easy to interpret and there is no pretesting required Disadvantages It doesn t provide a large amount of info you can see the difference but not the shape or function relating the independent and dependent variables o Pretest Posttest Design participants are randomly assigned to at least two groups and are tested on the key dependent variable twice once before and once after exposure to the independent variable Advantages It is simple Ensures that the two groups were equivalent at Disadvantages pretesting can change the participants spontaneous the pretesting behavior What is carryover and what are the six different types of carryover effects for within subject designs Carryover effects AKA order effects Exposure to a previous treatment affects performance in a subsequent treatment also called order effects This is an issue for within participant designs but it is not an issue for between participants designs Types of Carryover Effects 1 Learning Learning a task in the first treatment may affect performance in the second irrespective of the independent variable 2 Fatigue Earlier treatments may affect performance in later treatments because participants will get tired 3 Habituation Repeated exposure to a stimulus may lead to unresponsiveness to that stimulus to another before 4 Sensitization Exposure to a stimulus may make a subject respond more strongly 5 Contrast Participants will compare the current treatment to treatments that came 6 Adaptation Physiological changes over time based on experimental setting What are potential ways to deal with carryover effects Introduce breaks into the study to allow effect of previous treatment to wear off Practice trials to let participants get used to the new treatment and forget about old treatment If you have enough participants in your study you can include treatment order as an independent variable in your analysis This will let you know if significant carryover effects are present in your study Counterbalancing What different methods are there of counterbalancing and what are the advantages disadvantages of each Complete Counterbalancing Includes all of the possible treatment orders o Advantages All possible combinations of treatments are represented o Disadvantages The minimum number of subjects can get large fast Example 5 conditions 5 X 4 X 3 X 2 X 1 120 subjects needed Partial Counterbalancing Includes only some of the possible treatment orders o Latin Squares Design a formal system of partial counterbalancing that ensures that each condition appears in each position at least once Example A Latin Square for 5 conditions may look like this A B E C D B C A D E C D B E A D E C A B E A D B C Chapter 11 More on Experiments Confounding and Obscuring Variables Be able to describe the multiple threats to internal validity in class You won t need to list define them but you ll need to identify them and or be able to know the differences among them Internal Validity a result is internally valid when the independent variable caused the observed change in the dependent variable There are many different things that can threaten internal validity o History Events that occur between the dependent variable measurements in a repeated measures design pretest posttest Example study on informational propaganda in 1940 Measure attitudes read Nazi propaganda and then measure attitudes again But before they measure attitudes again France falls to the Nazis This effects the attitudes as well not just the propaganda o Maturation Changes in participations that occur over time during an experiment These changes could include actual physical maturation or tiredness
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