Stat 231 Final S10 Key

# Stat 231 Final S10 Key - Stat 231 Final Exam Spring 2010 I...

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1 Stat 231 Final Exam Spring 2010 I have neither given nor received unauthorized assistance on this exam. ________________________________________________________ Name Signed Date _________________________________________________________ Name Printed

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2 1. ISU Students Kotlers, MacFarland, and Tominson studied strength properties of wood joints. Stress at failure, y , was measured for joints of each of four types (in units of psi). Some summary statistics like those from the students' study are below. Type #1 Type #2 Type #3 Type #4 1 5 n 2 1 n 3 2 n 4 4 n 1 712.6 y 2 1169.0 y 3 929.5 y 4 1428.0 y 1 164.8 s 3 99.7 s 4 188.1 s Initially, consider only the information from the study about joints of Types #3 and #4, and assume strengths of such joints are normally distributed. a) Give two-sided 95% confidence limits for the difference in mean-stress-at-failure values for the two joint types. (Plug in completely, but you need not simplify.) b) Give two-sided 95% confidence limits for the ratio of standard deviations of measured stress at failure for the two joint types. (Plug in completely, but you need not simplify.) Now, consider all four joint types, under the assumption that strengths of joints of any fixed type are normally distributed and the variability of strengths is the same for each type. c) Give an estimate of the standard deviation of measured strength for any fixed joint type. 5 pts 5 pts 5 pts
3 d) Joint Types #1 and #2 are Butt joints and Types #3 and #4 are Lap joints. So, the quantity 12 34 11 22 is potentially of interest. Give 95% two-sided confidence limits for this.

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Stat 231 Final S10 Key - Stat 231 Final Exam Spring 2010 I...

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