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Stat 217 – Day 3 Topic 3: Drawing ConclusionsLast Time – Drawing ConclusionsActivity 3-2 (p. 36)Activity 3-2Slide 5Activity 3-4 (p. 40)Other examplesParameter vs. Statistic (p. 35)Slide 9Lab 1: Friend or FoeSlide 11Possible explanations“Simulation”Slide 14Stat 217 – Day 3Topic 3: Drawing ConclusionsLast Time – Drawing ConclusionsIssue #1: Do I believe the sample I have is representative of the population that I am interested in for this issue (generalizable)?Many possible sources of sampling biasVoluntary response, bad sampling frame, nonresponseNOT sample size…population???sampleActivity 3-2 (p. 36)(a) Observational units = studentsVariable = whether or not intentionally injured themselves(b) Population of interest = “college students”Sample = students that responded(c) sample size = 2875Activity 3-2(f) No, they only sampled from two universitiesThat are rather prestigious and have different stress levelsSo students probably more likely to self injureThat have wealthier students, supportive familiesSo students probably less likely to self injureNo, voluntary responseSo those who have experience, strong feelings more likely to respondIf embarrassed, less likely to respondGive main reason why, try to argue a particular direction (over or under estimate), make sure connect to the variable being measuredLast Time – Drawing ConclusionsIssue #2: Can I draw a cause and effect conclusion when comparing groups (causation)?Explanatory variable vs. Response variablesampleExplanatory Group 1Explanatory Group 2Response variableActivity 3-4 (p. 40)A confounding variable (p. 39) changes with the explanatory variable and possibly also affects the response variable, can’t distinguish whichObservational unitsSports SectionVariety of examplesExplanatory variable Response variablePerformance in courseStats studentsEarly time Later timeNon-athletesathletesNot necessarily confounding variables:• Some students study more than others (doesn’t differ between groups)• Instructor (doesn’t differ)• Easier to find parking in the morning (not clearly related to response)Other examplesCEOs are taller than non-CEOsShifts with Kristin Gilbert working saw higher death ratesActivity 3-5 (p. 41):Quebec children with more sleep at night are less likely to be obeseIn the late 1940s, polio cases increased with the consumption of ice cream and soft drinksParameter vs. Statistic (p. 35)Parameter is a number that describes (the variable in) a population63% of all voters that actually voted for Roosevelt (37% that voted for Landon)Average number of hours Cal Poly students slept last nightStatistic is a number that describes (the variable in) a sample57% of voters who indicated they would vote for Alf LandonAverage number of hours of students in this class that slept last nightActivity 3-2 (p. 36)(d) 17% is a statistic because it described the sampleWhat would the parameter be?The proportion of all college students that have injured themselves intentionallynumberpopulationvariableLab 1: Friend or FoeExperiment 1Lab 1: Friend or Foe14 of 16 infants picked the helperDoes this convince you that these infants are generally more likely to pick the helper than the hinderer based on the videos?Discuss with neighborJot down ideasPossible explanationsThese infants genuinely prefer the helper toyThese infants do not genuinely prefer the helper toy but we happened, by chance alone, to get a large majority picking the helper in our sample.We can investigate this second case – If it is the case there is no preference, how often get 14 out of 16 picking the helper“Simulation”Instead of working with infants, we will assume the infants behave like a coin toss. Assuming same probability for each infantToss your coin 16 times, to represent the 16 identical infants, and record the number of headsIs it surprising to get 14 heads when we know heads and tails are equally likely?What conclusion does this point to?For Thursday (Library)Pre-lab for Lab 1 by noonWill email back feedbackDon’t need to bring your textDo bring a USB for saving your work to continue outside of classSit with a partnerFor Monday: Activity 4-1


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Cal Poly STAT 217 - Drawing

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