The survey continued to be administered in respondents' households. One major change in 1999 was the implementation of a computer-assisted administration, which allowed respondents to enter their answers to sensitive questions directly into the computer. The survey used a State-based sampling plan that permits the estimation of substance-use patterns for every State. A larger overall sample allows for more precise estimation of use among various age groups and racial/ethnic groups. The percentage of regular (past month) illicit drug users by age was 11 percent among the 12 to 17 age group, 17 percent among those 18- to 25-years-old, and 4 percent in the 26-year-old and older group. Less than half of current drug users in each age group used an illicit drug other than marijuana. Twenty percent of the population reported past-month binge alcohol use (defined as drinking five or more drinks on one occasion), including 38 percent of those in the 18-25 age group. An estimated 3.6 million Americans (approximately 2 percent of the population 12 and over) were dependent on illicit drugs, and an estimated 8.2 million (4 percent) were dependent on alcohol, according to survey estimates. Twelve percent of the population aged 18 to 25, 6 percent of those 12 to 17, and 3 percent of those 26 and older were estimated to be dependent on drugs or alcohol. Early initiation into marijuana use was associated with much higher rates of adult drug dependence. Nine percent of adults who first used marijuana at age 14 or earlier were dependent on illicit drugs, compared to only 4 percent of adults who first used marijuana between the ages of 15 and 17, and less than 2 percent of adults who first used marijuana at age 18 or older. 1 table