A device has been developed to detect the presence of cataract more 
accurately by characterizing the average greyness of the eye over an 
image of 10,000 pixels on a scale from 1 to 256.  Six subjects with a 
cataract in one eye only are measured.  What can you say about the 
mean differrence in grey levels of eyes with and without cataract?  
What is your opinion of the measuring device?