Myopia (i.e., nearsightedness) is a visual condition that affects over 100 million Americans. Two treatments that may slow myopia progression are the use of (1) corneal reshaping contact lenses and (2) bifocal soft contact lenses. In Optometry and Vision Science (Jan., 2013), university optometry professors compared the two methods for treating myopia. A sample of 14 myopia patients participated in the study. Each patient was fitted with a contact lens of each type for the right eye, and the peripheral refraction was measured for each type of lens. The differences (bifocal soft minus corneal reshaping) are shown in the following table. (These data are simulated, based on information provided in the journal article.)

a. Find measures of central tendency for the difference measurements and interpret their values.
b. Note that the data contain one unusually large (negative) difference relative to the other difference measurements. Find this difference. (In Section 2.7, we call this value an outlier.)
c. The large negative difference of -8.11 is actually a typographical error. The actual difference for this patient is -0.11. Rerun the analysis in part a using the corrected difference. Which measure of central tendency is most affected by the correcting of the outlier?

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Peripheral refraction differences -0.79 -0.39 -0.41 -0.15 -0.81 0.05 -0.68 -0.80 -0.63 -8.11 -0.32 -1.13 0.01 -1.11