The DMP's task is accomplished by making undercover heroin purchases that are then analyzed. During 1993 the DMP collected samples in 20 metropolitan areas, yielding 631 exhibits, 597 of which contained heroin; 590 exhibits were used in the testing. These exhibits averaged 35.8 percent in purity, with a low of 0.8 percent in Miami and a high of 98.3 percent in Boston. This average purity is slightly lower than the 37.0 percent reported in the DMP for 1992. A total of 96 exhibits were of Southeast Asian (SEA) origin, with an average purity of 32.2 percent. SEA heroin was predominant in the northeastern United States and was the primary type of heroin available in cities where Nigerian traffickers were well-established. A total of 37 exhibits were of Southwest Asian origin, with an average purity of 47.2 percent. A total of 160 exhibits were of Mexican origin, with an average purity of 27.8 percent. This heroin was the predominant type available in the western and southwestern United States. A total of 48 exhibits were of South American origin, with an average purity of 59.3 percent. A total of 249 exhibits were of unclassifiable origin, with an average purity of 36 percent. Data show the average cost per milligram of pure heroin by source area. The tables show data for each of the 20 metropolitan areas from which samples were taken.