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Lecture 8

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PSYC 2130
Krista Phillips

Lecture Seven (Class Eight): • An independent variable, by definition, must have at least two levels • There are two general possibilities in experimental design o Between-subjects design o Within-subjects design  Also known as nested design or repeated-measures design • Siall and Ostrove: Attractiveness on jail sentencing o Two between subjects:  Two crimes • Burglary o Barbara breaks into neighbours apartment, steals 2000 • Swindling o Barbara meets a stranger in a bar, convinces him to invest in a non-existent company  Two levels of attractiveness • Attractive • Unattractive  Control group: • No photograph o Harsher sentence when swindling and attractive, lower sentence when burglary and attractive o Has to be a between subject, because no one can be naive to the attractiveness o Problem with this?  Confounding variable: differences between conditions may be due to individual differences  Also, must have at least 30 people in each group for between subjects  No way to know that the obtained results are due to manipulation or something else • Random assignment to remedy this • Within subjects: needs fewer subjects which is easier. Sometimes it is the only reasonable choice o pros  Expose all subjects to all manipulations  Eliminates the equivalent groups problem  Convenient because you need less people o Cons:  Order or sequencing effects  Time effects • You can control these by counterbalancing o In some experiments where subjects are only tested once in each condition, the best thing to do is use complete counterbalancing. Every possible sequence will be used at least once o When the number of levels is higher than two, its harder to do this  X! (x-factorial) o If there are a large number of conditions, one effective strategy to employ is a Latin Square Design  every con
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