The interrelationship of the standard deviation, the coefficient of variation, and the compressive strength of concrete was studied on 32 building sites and 14 ready-mix plants, with more than 30,000 observations analysed. While the standard deviation was found to be practically strength-independent when overall values were considered, the coefficient of variation showed such independence in its daily values. It was further concluded that formal compliance with a set of nominal requirements does not necessarily secure the expected strength variation, in which respect the actual level of supervision is the dominant factor. The average overal standard deviation was of the order of 60 kg. per sq. cm. and the daily coefficient of variation—of the order of 10%, irrespective of nominal conditions of concrete production.