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Unformatted text preview: 1 Assignment After this lecture, start Assignment #4. Its in the course binder. 2 Compared to what? Lecture 10 Hypothesis test for a mean 3 Overview What is a hypothesis? How do you compare a hypothesis to your data using confidence intervals using sampling distributions 4 1. What is a hypothesis? 5 What is a hypothesis? Hypothesis = Claim about a parameter Today: claim about the population mean Y . E.g., the average starting salary of all sociology BAs Two alternatives: null hypothesis (H ) a neutral claim research hypothesis (H 1 ) onetailed or twotailed 6 Twotailed hypotheses Does journaling affect depression Effect has two possible directions Journaling works through issues depression lifts Journaling dwells on problems depression deepens Let Y be average change in depression after journaling. Null hypothesis: journaling does not affect depression H : Y = 0 Research hypothesis: journaling affects depression : 1 Y H 7 Onetailed hypotheses Do smokers have lighter babies? Expected effect has one direction In general population, average birthweight is say 7 pounds Let Y be average weight of smokers children. Research hypothesis: smokers have lighter babies H 1 : Y < 7 Null hypothesis: smokers have average babies (or even heavy babies) H : Y = 7 (or H : Y >= 7) both forms are useful 8 One tail or two? Key: Does the research hypothesis specify direction? One tail H 1 : Y < 7 H : Y = 7 (or H : Y >= 7) Two tails : : 1 = Y Y H H 9 2. Hypothesis tests 10 Hypothesis test: Definition Given evidence from a sample , we test (decide whether to reject) the null hypothesis about the population 11 Research question Should you finish college? Should you delay graduation for parttime work? UPS offers $9 an hour plus $3000 a year for books and tuition $9/hour X 2000 hours/year + $3000 = $21,000 per year Will you do better than that when you graduate? 12 Null vs. research hypothesis: Symbols Let Y be the starting salary for a sociology BA 2200 Y is the average for the population of sociology BAs H 1 : Y > $21K The average is more than what you could make at UPS....
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 Spring '09

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