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Stat 321 – Day 3AnnouncementsLast Time – Measures of CenterLast Time – “Test” for outliersOld Faithful inter-eruption timeExample 1: Hypothetical Quiz ScoresExample 2: Hypothetical Quiz ScoresWhat is “variability”?Example 2: MatchingExample 3: More MatchingFor MondayFor tomorrowStat 321 – Day 3Measures of SpreadAnnouncementsLab questions?NHL: $209.78 MLB: $147.72NFL: $322.82 NBA: $244.48HW 1 (due Tuesday) questions?Reminder: Office Hour today 1-3Last Time – Measures of CenterMean = average all valuesBut watch for outliers (not resistant)Median = “middle” observation“Typical” valueNotes:“mode” usually not so useful with quantitative dataCenter may not be most relevant to study questionPercentile = value with p% of data below itMedian = 50th percentileLower fourth = 25th percentileLast Time – “Test” for outliers1.5fs criterionAny values more than 1.5fs beyond lower fourth or upper fourth is flagged“Extreme” outlier if more than 3fs awayModified boxplotOld Faithful inter-eruption timeExample 1: Hypothetical Quiz ScoresPredictionsFind the fourth spread for each classMedian = 15th observation = 5 pointsLower fourth = median of bottom 15 observations= 8th observation = 2Upper fourth = 8Fourth spread = 6Example 2: Hypothetical Quiz ScoresPredictionsFind the fourth spread for each classClass A Class B Class C Class D Class E Class F 3.0 6.0 4.0 8.0 0.0 2.0Standard deviationsClass A Class B Class C Class D Class E Class F2.209 3.047 2.631 3.328 2.760 1.333What is “variability”?Variability considers the horizontal “width” of the distribution, how far values tend to lie from the centerNot “bumpiness/uneveness” or “variety”Example 2: MatchingA – 1, symmetric D – 2, symmetric with much larger spread F – 6, symmetric and closer to centerE – 3, positively skewed B – 4, positively skewed with more spreadC – 5, negatively skewedExample 3: More MatchingHeightsNumber of siblingsLast haircut costChange in pocketsGuesses of my age1 2345For MondayContinue working on Lab 2 and HW 1 (Tues)Feel free to email questionsFinish reading Ch. 1, start Ch. 2 (2.1, 2.2)Review Set Operations and Properties handoutFor tomorrowMeet in Library computer lab!111B (straight back, turn right, then left)Turn in Lab 1 as enterBring USBSit with a partnerTwo apologiesBe nice to guest instructor…No Friday office hour this


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Cal Poly STAT 321 - Measures of Spread

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