A helpful first step is to see if the observed association is specific to the disease under study. For it is intuitively recognized that if the association is specific there is a greater likelihood of an etiologic relationship than if it is not. An association which is nonspecific, diffuse, manifesting itself in a large and varied number of diseases, increases the probability that the observed differences are due to extraneous factors which differentiate the self-formed groups. It then becomes crucial to study in great detail the differences and similarities of persons with the environmental factor under study and those without it.