Drug arrests are classified into four categories: 1) heroin or cocaine and their derivatives, 2) marijuana 3) synthetic or manufactured drugs like fentanyl and 4) other dangerous non-narcotic drugs like barbiturates.

In 2018, there were 663,367 arrests involving marijuana, up from 659,700 in 2017, nearly 92 percent of them for possession. The F.B.I.’s crime data includes only the top charge for each arrest, so if a suspect is found with drugs while being arrested on a more serious charge, the drug possession would not be counted in the agency’s statistics.

“I always caution people to read the U.C.R. data as an approximation because it’s imperfect,” said Tess Borden, a staff attorney with the American Civil Liberties Union who worked on a report published by the A.C.L.U. and Human Rights Watch in 2016: Every 25 Seconds: The Human Toll of Criminalizing Drug Use in the United States.

According to New York State’s Division of Criminal Justice Services, there were 75,897 arrests for drug felonies and misdemeanors in New York in 2018, which includes any arrest where fingerprints were taken. About 35 percent of those arrests involved people who were identified as white; 37 percent as black; 25 percent as Hispanic; and 2 percent as Asian. The remainder were listed as other/unknown. (In New York State, blacks make up 18 percent of the population, and Hispanics 19 percent.)

“We know from national survey data that people of all races use drugs in their adult lifetimes at approximately the same rates,” Ms. Borden said. “So the fact that we have great variances in who is arrested tells us about police priorities.”

In 2021, the F.B.I. plans to begin using its National Incident-Based Reporting System to track crime data, which has more detail about a greater number of crimes.

This reporting system also contains information about the quantity of drugs involved in an arrest. Analyzing 700,000 drug arrests using this data for 2004, 2008 and 2012, the authors of the “Sharks and Minnows” paper found that about 40 percent of those arrests were for possessing or selling a quarter of a gram or less of drugs. And 20 percent were for possessing or selling drugs weighing between 0.25 grams and one gram. (A packet of Splenda sweetener weighs one gram.)