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PS295 Chapter Eight Summary.docx

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Wilfrid Laurier University
Roger Buehler

PS295 Chapter Eight Summary Basic Issues in Experimental Research Manipulating the IV Independent Variables  Researchers manipulate IV to assess their effects on participants behaviour  Must have two or more levels  Sometimes different levels of IV are referred to as experimental conditions  Types of IVs include: (1) environmental manipulations – involve experimental modifications of participants physical or social environment (2) instructional manipulations – vary the IV through the verbal instructions that participates receive (3) invasive manipulation – involve creating physical changes in the participants body through surgery or administration drugs  Experimental and control groups - the caffeine and memory study: participants who receive non zero level of the IV compose the experimental groups and those who receive a zero level of the IV make up the control group  Some researchers use pilot test to test the levels of the IV’s; the purpose of this is not to see whether the IV produce hypothesized effects on the participants behaviour but to ensure that the levels of the IV are different enough to be detected by participants  Some use a manipulation check which is a question that is designed to determine whether the IV was manipulated successfully  Subject variables reflect existing characteristics of the participants Dependent Variable  Is the response being measured in the study  They typically involve either observations of actual behaviour, self-report measures or measures of physiological reactions Assigning of Participants to Conditions Simple Random Assignment  Involves placing participants in conditions in such a way that every participant has an equal probability of being placed in any experimental condition  For example -> if we have an experiment with only two conditions; you can flip a coin  It ensures participants in groups do not differ Matched Random Assignment  The researcher obtains participants scores on a measure known to be relevant to the outcome of the experiment  It is a pretest measure of the DV  Then participants are ranked on this measure from highest to lowest, then matches participants by putting them in clusters or blocks of size k where k is the number of conditions Repeated Measures Design  When participants are assigned to each of the conditions in an experiment, as in simple and matched random assignment this is called randomized groups (or between-subjects or between-groups design)  A single group of participants serves in all conditions the experiment  Within subjects designs in which we are interested in differences of behaviour across conditions within a single group of participants  This eliminates the needs for random assignment because every participant is tested under every level of the IV  Advantages of within-subjects design: (1) it is more powerful than a between-subjects; the power refers to its ability to detect effects of the IV (2) they require fewer participants because everyone is used in every conditions  Disadvantages… (1) since each participant receive all levels order effects can arise when participants behaviour is affected by the order in which they participate in the various conditions (2) practice effects occur when the participants performance improves merely because they complete the DV several times (3) fatigue effects (4) sensitization; after receving several levels of the IV participants may begin to realize the hypothesis (5) carryover effects occur when the effect of the particular treatment condition persists even after the condition ends; they occur when the effects of one level of the IV are still present when another level of IV is introduced  To guard these possible order effects researchers use counterbalancing which involves presenting the levels of the IV in different order to different participants  A latin square design is where each condition appears once at each ordinal position and each condition precedes and follows every other condition once Experimental Control  Refers to eliminating or holding constant extraneous factors that might affect the outcome of the study
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