R. SYM Sym Statistics

The limits on size in BS 3921 are based on information gathered approximately 40 years ago. To allow the appropriateness of these limits to today’s products to be assessed, a study of the dimensional deviations of clay bricks was carried out. Random samples were taken of the bricks produced at 37 UK brickworks. The lengths, widths, and heights of bricks were measured individually and by the 24 bricks-in-a-row method. The size of the dimensional deviations varies widely between products. Clamp-fired products for example, have relatively high short-term variability and some tunnel kiln products have relatively high long-term variability. There is a relation between the standard deviation of length, width and height measurements from which it follows that dimensional tolerances for clay bricks should be proportional to the square root of the work size. The data also showed that acceptance criteria for dimensional deviations may be based on the ‘Normal’ assumption.