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# stat420_hw7 - STAT 420 Homework 7 Spring 2014 Due April 4...

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STAT 420 Spring 2014 Homework 7: Due April 4 by 7:00pm Exercise 1 (revisited) Chemists often use ion-sensitive electrodes (ISEs) to measure the ion concentration of aque- ous solutions. These devices measure the migration of the charge of these ions and give a reading in millivolts (mV). A standard curve is produced by measuring known concentrations (in ppm) and fitting a line to the millivolt data. The below table gives the concentrations in ppm and the voltage in mV for calcium ISE. i ppm mV i ppm mV i ppm mV 1 0 1.72 7 75 2.40 13 150 4.47 2 0 1.68 8 75 2.32 14 150 4.51 3 0 1.74 9 75 2.33 15 150 4.43 4 50 2.04 10 100 2.91 16 200 6.67 5 50 2.11 11 100 3.00 17 200 6.66 6 50 2.17 12 100 2.89 18 200 6.57 (a) Plot the points mV ( y ) versus ppm ( x ). Overlay the least-squares (linear) regression line, and comment on whether the line seems to fit the data. (b) Fit a quadratic model. Overlay the least-squares (quadratic) regression line; does it seem to provide a better fit? (c) Fit a cubic model. Overlay the least-squares (cubic) regression line; which model (linear, quadratic, or cubic) do you choose and why? Exercise 2 A survey was conducted to study teenage gambling in Britain. (Ide-Smith & Lea, 1988, Journal of Gambling Behavior, 4, 110–118) The data is stored in the data frame teengamb (library

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