Macular degeneration is the most common cause of blindness is people older than 60 years. One variable thought to be related to a type of inflammation associated with this disease is level of a substance called soluble Fas ligand (sFasL) in the blood. The accompanying table contains representative data on x 5 sFasL level for 10 patients with age-related macular degeneration. These data are consistent with summary quantities and descriptions of the data given in the paper “Associations of Plasma-Soluble Fas Ligand with Aging and Age-Related Macular Degeneration” (Investigative Ophthalmology &

Visual Science [2008]: 1345-1349). The authors of the paper noted that the distribution of sFasL level was skewed and recommended a cube-root transformation. The cuberoot values and the normal scores for a sample size of 10 are also given in the accompanying table.

x Cube Root of x Normal Score 0.90 1.539

a. Construct a normal probability plot using the untransformed data. Does the normal probability plot appear linear or curved?

b. Compute the correlation coefficient for the (normal score, x) pairs. Compare this value to the critical r value from Table 7.2 to determine if it is reasonable to consider the distribution of sFasL levels to be approximately normal.

c. Construct a normal probability plot using the transformed data. Does the normal probability plot ap-

pear more linear than the plot for the untransformed data?

d. Compute the correlation coefficient for the (normal score, transformed x) pairs. Compare this value to the critical r value from Table 7.2 to determine if it is reasonable to consider the distribution of transformed sFasL levels to be approximately normal.