The average stocks of the eight blood groups, classed by ABO and Rhesus typing, are as follows: Blood group Average number of days stock (by year) and work in progress O negative 7.6 O positive 8.2 A negative 7.8 A positive 9.5 B negative 7.8 B positive 10.5 AB negative 9.0 AB positive 17.5 Please note that a similar amount of stock is normally held by NHS Scotland hospital blood banks...