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UW STAT 220 - Exam Guide

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STAT 220 Midterm May 1, 2009Name (print):Section:This is the Midterm for Stat220. There are 4 questions for a total of 50 points. You have50 minutes to work on this exam.Exam rules:1. You must keep the quiz closed and are not allowed to write until it isannounced that you can start the exam.2. This is a closed book exam. You are allowed to bring one letter-sized sheet of notes.3. You may use a calculator. Cell phones may not be used.4. You will not get full points for an answer without explanation. So make sure to showyour work, and explain all your answers.5. Write your name and section on the exam, and sign the honor statement below, certi-fying that the exam is completely your own work.6. Code of conduct: you must do this exam completely independently. Students suspectedof cheating or dishonest practices will be reported to the Vice President of StudentAffairs. Such practices include, but are not limited to (i) making use of books orlecture notes, (ii) communicating with other students in any way, (iii) looking at thework of other students, and (iv) purposely e xposing your work to the view of otherstudents.Good luck!!Honor statement: I certify that this exam is completely my own work, and that I followedthe code of conduct as outlined in point 6 above.Signature:Question: 1 2 3 4 TotalPoints: 12 13 17 8 50Score:1 of 8STAT 220 Midterm May 1, 20091. This question is about confounding factors.(a)(4) Give the definition of a confounding factor.(b)(4) Explain why we don’t have to worry about confounding factors in large double-blindrandomized controlled experiments.(c)(4) (Hypothetical) An investigator took a simple random sample of 200 hospitals inthe US, and found that the average death rate in large urban hospitals was twiceas high as the average death rate in small country hospitals. Mention a possibleconfounding factor that could explain the difference in death rates between thesetwo types of hospitals, and argue carefully how it could explain this difference.(Note: the investigator also found that the large urban hospitals tended to havebetter medic al equipment and better educated doctors and staff.)2 of 8STAT 220 Midterm May 1, 20092. This question consists of three unrelated parts:(a)(4) Which implies a stronger linear relationship: a correlation of +0.4 or a correlationof −0.6? Briefly explain.(b)(5) Compute the SD of the following list: 4, 4, 3, 3, 1, 33 of 8STAT 220 Midterm May 1, 2009(c)(4) Fill in the blanks, using the options below, and give examples (or explanation) toshow that you picked the right answers:• The SD of a list is 0. This means ...• The r.m.s. size of a list is 0. This means ...Options:(i) there are no numbers on the list(ii) all the numbers on the list are the same(iii) all the numbers on the list are 0(iv) the average of the lis t is 04 of 8STAT 220 Midterm May 1, 20093. Below are summary statistics on the men’s gold medal performance in the Olympics,from 1900 to 2008.• Gold medal performance in 200m sprintMean = 20.7 seconds, SD = 0.9 seconds• Gold medal performance in 400m sprintMean = 45.9 seconds, SD = 2.1 secondsThe correlation coefficient is r = 0.95.(a)(6) In 1996, Michael Johnson became the only male athlete to have won both the 200mand the 400m at the same Olympics. His 400m winning time has a percentile rankof 96%. Predict the percentile rank of his 200m winning time.5 of 8STAT 220 Midterm May 1, 2009(b)(6) Johnson’s 200m winning time was 19.3 se conds. Predict his 400m winning time.(c)(5) Johnson’s true 400m winning time was 43.5 seconds. How large is the predictionerror? How likely is it to get a prediction error that is larger in magnitude?6 of 8STAT 220 Midterm May 1, 20094. Investigators are studying the relationship between income and education, for women inthe U.S. age 25-54 who are working.(a)(4) Investigator A computes the correlation between income and education for all thesewomen. Will this correlation be positive or negative? Explain your answer.(b)(4) Investigator B looks at each state separately, computes the average income and aver-age education of that state, and then computes the correlation coefficients for the 50pairs of state averages. Which investigator gets the higher correlation, investigatorA or B? Or should the correlations be about the same? Explain.7 of


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UW STAT 220 - Exam Guide

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