THE Bush administration and its critics are attempting to divine something substantive in the mixed readings of 1991 drug abuse statistics - particularly an increase in cocaine use. But experts on the subject caution that one year's data says little about the actual course of the war on drugs.While cocaine use rose in 1991 for the first time since the 1985 peak, overall illicit-drug abuse continued a 12-year decline, according to data from the annual National Household Survey on Drug Abuse released last week by federal officials. Cocaine abuse rose among those over 35, the unemployed, blacks, and high school dropouts, but declined among youth and the middle-class, explains Bob Martinez, director of the office of National Drug Control Policy. His interpretation of the data is that drug-abuse prevention programs are working to keep youth and the middle-class off drugs. But hard-core users are more difficult to reach, he says, adding that Congress had not fully funded the administration's plans for treatment programs. Meanwhile, critics called the household survey data showing an 18 percent increase in cocaine use in 1991 an indication the administration's drug war is not working. But experts who study drug abuse are less likely to conclude anything from the mixed statistics. "From a public policy point of view, we reach consensus in the middle class of absolute opposition to cocaine and we reach that quicker than the decline in use," explains David Musto, a Yale University professor of psychiatry and the history of medicine. "The decline in drug use is very extended; it can take 10 to 15 years." Peter Reuter, a RAND Corporation analyst, concurs: "The declines from 1988 to 1990 looked surprisingly high. But now there'll be a much slower decline, and when you measure year to year, an uptick in the 35-plus [age group] is absolutely meaningless." Dr. Musto, whose book, The American Disease, points to parallels in the current cocaine epidemic and the first one that ran in the US from the 1880s through the turn of the century. After initial acceptance among the middle class as a legal and socially acceptable mental stimulant - much as it was socially acceptable among the middle class in the 1970s and 1980s - cocaine was later outlawed, Musto explains. And as the middle class rejected it in the early 1900s, cocaine use became linked in the public mind with gangs, prostitution, and violence, he says.