Essentially, to secure the maximum output of coal from the minimum length of face with due regard to safety and development. To measure the degree of concentration at a colliery the following data are collected: (1) the total length of coalface; (2) the total length of main haulage roads; and (3) the total output. In general, the greater the dispersion of the workings, the greater the manpower employed and the higher the costs of production.