PSYCH 303 Lecture 6: Experiments
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Department
Psychology
Course Code
PSYCH 303
Professor
Colleen Seifert

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March 6, 2017 Experiments • What causes obesity? • Sugar consumption is correlated with fat consumption and total calories • To claim that sugar is the cause, you would need a study that controls for these variables • According to the sugar industry… • Sugar as a percentage of total calories has remained stable but the percentage of fat in our diets has increased • When you control for total calories consumed, sugar does not explain any additional variance in weight gain • Based on this statement, the sugar industry relied on associations at a to show that sugar didn’t explain any unique variance in weight gain • Causal claims • What is necessary in order to make a causal claim? • Temporal precedence • Internal validity • Manipulation of independent variable • Ludwig et al. (1999)’s experiment • Sample of 12 obese teenage boys • Procedure • Check into hospital at 6pm • Eat low-sugar dinner and bedtime snack Lights out at 10pm • • Same test meal given as breakfast at 7am and lunch at noon; remain in bed in-between • Manipulate sugar content of test meal • Sugar was the independent variable • Independent variable was manipulated within groups across three separate testing sessions • Non-representative sample • What is the advantage of a uniform sample? • Does it have external validity? • Control variables • Controlled for activity level, sleep, prior food intake Measure dependent variables • • Feelings of hunger • After breakfast, participants asked to rate hunger every 30 minutes • Later eating behaviors • After lunch, participants invited to ask for more food whenever they were hungry • Caloric intake tracked for five hours • Repeated measures • Experiment shows that higher sugar leads to increased feelings of hunger and eating, which leads to higher caloric intake • Experiments—what and why • What makes a study an experiment? At least one manipulated variable (with at least two levels for comparison) • • At least one measured/outcome variable • Why do experiments? • Exert more control over variables • Helps establish causation Limitations—why not do experiments? • • Ludwig et al. (1999) • Sample of 12 obese teenage boys • Procedure • Check into hospital at 6pm • Eat low-sugar dinner and bedtime snack • Lights out at 10pm • Same test meal for breakfast at 7am and lunch at noon; remain in bed in-between • Manipulate sugar content of test meal • Measure dependent variables • Feelings of hunger Later eating behaviors • • Controlled study shows eating more sugar leads to more hunger and more eating • Lab study shows fat kids who lay in bed and eat more sugar then feel hungry and eat more… it’s them, not you! • Carney, Cuddy, & Yap (2010) Power Poses • Laboratory study shows power poses lead to higher power ratings • Hypothesis: nonverbal expansiveness (vs. contractiveness) will increase subjective feelings of power • Between-subjects design • Body stance as independent variable • Feelings of power and being “in-charge” as dependent variable Cover story: story about physiological recording above and below heart level • • Classroom replication study shows power poses make no difference in power ratings • Hypothesis: nonverbal expansiveness (vs. contractiveness) will increase subjective feelings of power • Within-subjects design • Body stance as independent variable • Feelings of power and being “in-charge” s dependent variable • Cover story: story about superhero character responses • Experimental designs • Independent-group design • Allow for random assignment of participants to conditions Randomizes the condition each person gets • • Desystematizes the type of people who end up in each group • Between-subjects design • Participants are assigned to different conditions (levels) of the independent variable • Eliminates possibility of third variable confound • Advantages • Every subject does just one condition • No order effects (practice, fatigue, carryover (sequence)) • Exposure to only one level of independent variable • “Uninformed” subject • Disadvantages Groups may not be equivalent (especially if small) just by chance • • Less statistical power to notice differences • Need more participants/bigger sample size • Gollwitzer & Melzer (2012) Hypothesis: after playing an amoral video game, participants will want “hygiene • products” • “Macbeth effect” = need to physically cleanse self when morality threatened • Convenience sample of 76 college students random
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