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Unformatted text preview: 1 Assignment • After this lecture, start Assignment #4. – It’s 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 μ 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 μ Y be average change in depression after journaling. • Null hypothesis: journaling does not affect depression H : μ Y = 0 • Research hypothesis: journaling affects depression : 1 ≠ Y H μ 7 Onetailed hypotheses • Do smokers have lighter babies? – Expected effect has one direction • In general population, – average birthweight is say 7 pounds • Let μ Y be average weight of smokers’ children. • Research hypothesis: smokers have lighter babies – H 1 : μ Y < 7 • Null hypothesis: smokers have average babies (or even heavy babies) – H : μ Y = 7 (or H : μ Y >= 7) • both forms are useful 8 One tail or two? • Key: Does the research hypothesis specify direction? • One tail – H 1 : μ Y < 7 – H : μ Y = 7 (or H : μ Y >= 7) • Two tails : : 1 = ≠ Y Y H H μ μ 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 μ Y is the average for the population of sociology BAs • H 1 : μ Y > $21K – The average is more than what you could make at UPS....
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 Spring '09
 Normal Distribution, new sociology BAs

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