Not more than 15 percent, by count, of the heads of lettuce in any one container or bulk lot may be below the requirements which are prescribed by this article, but not to exceed 5 percent, shall be allowed for decay or slime. Individual containers in any lot may, however, contain not more than 1 ½ times the tolerance which is specified, if the percentage of defects in the entire lot averages within the tolerance specified, as determined by inspection of a representative sample.