Biostatistics PHC 6065

Ophthalmology

A camera has been developed to detect the presence of a cataract more accurately. Using this camera, the gray level of point (or pixel) in the lens of the human eye can be characterized into 256 gradations, where a gray level of 1 represent black and a gray level of 256 represents white. To test the camera photographs were taken of 6 randomly selected normal eyes and 6 randomly selected cataract eyes (the two groups consist of different people). The median gray level of each eye was computed over the 10,000 pixels in the lens. The data are given in the table below.

Table 8.16 Median gray level for catarctous and normal eyes

Patient number Catarctous median gray level Normal median gray level

1 161 158

2 140 182

3 136 185

4 171 145

5 106 167

6 149 177

x 143.8 169.0

s 22.7 15.4

8.34- What statistical procedure can be used to test whether there is a significant difference in the median gray levels between cataractous and normal eyes?

8.35- Carry out the test procedure mentioned in problem 8.34, and report a p-value

8.36- Provide a 99% CI for the mean difference in median gray levels between cataractous and normal eyes.