Document e5nqoNgOe0q7EZMZB5DLX9erq

all we could hear, but very low certified incidence. We hoped there to get a real contrast between the conditions In bituminous and anthracite mines in South Wales and the general relationship between dustiness and rank out side South Vales, to get mines which were really dusty but had low rank coals, and those are the two. Now, this slide is attributed to Doctor Cochran, jthB percentage of the underground-surface populations wnich he has managed to X-ray in each of those mines are given there, and the only two cases, in this mine here and here, in which he was. not fully in charge of the survey, there were early surveys, is the proportion below ninety-eight percent. The important aspect of that i's shown in two figures -- next slide. At these two collierys where we didn't get ninety-eight percent of the population, you will r see that the proportion varies with age. The statisticians! found difficulty in the casually young, and in the equally ; old* .The middle-age you get easier. If you don't get | I ninety-five percent you get seventy-five percent; both of these collierys you got over seventy-five percent of the j- population, but youfve got a distorted sample. It is es sential to get a hundred percent population. , The next slide -- and one colliery where he got a hundred percent. Doctor Cochran analyzed the amount of