METHOD OF SAMPLING.

The Ministry of Fuel and Power keeps a record of the names and addresses of all men certified by the Silicosis Board to be suffering from pneumoconiosis. This record was used for the selections of a sample of 796 men who had been certified up to and including December 1944. The method used was as follows:

(a) Each man whose name and address appeared on the record was given a number and this was punched on to a Hollerith card with his age, the year in which he was certified, his disability category. whether he was alive or dead, and the district in which he been certified In all, 6,531 cards were punched.

(b) These cards were then divided into two packs, one pack relating to the men who were thought to be alive and the other to the men who were known to be dead. Every 7th card selected from the pack of cards representing the men who were alive, and a further 14 cards selected from the lower age groups to increase their representation. This gave a total of 796 addresses. Every 7th card was also selected, from the pack representing the dead, giving a sample of 15 men who were known to the dead. A list of substitutes was selected in the same way.

(c) A letter was sent to each man who was thought to be alive asking if the address to which the letter had been sent was the correct one. In this way the majority of men were traced but a further 119 were found to be dead.

(d) In order to increase the sample of living men to the original figure of 796 the list of substitutes was used. Substitutes for each of the dead men were selected which were, as nearly as possible, in the same disability category, and the same area of the coalfield as the dead men had been.

(e) Each interviewer was given a list of names and addresses including men both alive and dead. The main schedule attached to this report was used to interview the living and a very short schedule was addressed to the surviving relatives of the dead, in most cases their widow.

The following is an analysis of what happened to the original sample of 796 names and addresses.

A sufficient number of substitutes, 136, was interviewed to bring the sample of surviving men with pneumoconiosis up to 764, 18% of the sample on which the following report is based were, therefore, substitutes.

The relatives of 129 of the 151 men known to be dead were also interviewed.

Comparisons of the original sample selected with the sample obtained indicate that the effects of substitution were small.

The only remaining comparison which can be made between the original sample and the sample obtained is less valid than those made above because part only of the original sample can be quoted. The Pneumoconiosis Research Unit gave each man interviewed, or each man whose relatives were interviewed, an X-ray category based on his original X-rays. Thus, of the 796 originally selected, 729 were given an X-ray category. This total includes 628 who were alive at the time of interview and 101 who were found to be dead, but whose relatives were interviewed.

If the 129 known dead for whom records were obtained are included with the 729 originals somewhat greater differences appear between the sample drawn and the sample obtained.

The sample obtained appears, therefore, to include 5% more in the lower X-ray categories than the original sample plus the dead, the nearest approach to the X-ray categories of all men certified that can be obtained. On the whole the differences between the original sample and the sample obtained are small, however, and unlikely to bias greatly the results given in the following pages.