Author

R. C. DE VEKEY and R. LOVEGROVE Building Research Establishment, Garston.

Abstract

Most UK standards for masonry units measure compliance with strength specifications using the mean result of destructive tests on a sample of 10, randomly selected from a batch or lot. Other countries in Europe use different sample sizes for example, 6 in Germany and 20 in Holland for some products. On the basis that the distribution of strengths for a batch of such products can be assumed to be normal (Gaussian) the size of the sample required to give a producer’s risk of 1 in 40 has been calculated as a function of the variability of the population and the difference between the specified value and the mean of the batch (fi—X). This indicates that a sample size in the range 5-10 is more logical than arbitrary since the response curve for samples in excess of 10 is very sensitive to changes in variability and i-t -X and could give control problems if exploited.