Low-level arrests for marijuana possession in New York City increased for the seventh straight year in 2011, according to a study released Wednesday — despite a September memorandum from the police commissioner that reminded officers to follow the letter of the law and not arrest people with the drug unless they have it in plain view.

Though arrests dropped significantly after Commissioner Raymond W. Kelly’s memorandum, an increase of over 6 percent during the first eight months of the year more than offset the decline, according to the analysis, conducted by a Queens College sociology professor and released by the Drug Policy Alliance, an advocacy group critical of police marijuana-arrest policies.

The year-end arrest total was 50,684, up 0.6 percent from 2010, the study found, constituting more arrests than in the entire 19-year period 1978 to 1996 combined. Marijuana possession was once again the largest arrest category in the city last year, and the arrests cost the city about $75 million, said Harry G. Levine, the sociologist who did the analysis.

The high numbers of marijuana arrests under the Bloomberg administration have been linked by critics to the police’s stop-and-frisk practices and disproportionate enforcement against blacks and Hispanics.

While state law makes possession of less than 25 grams of marijuana an arrestable misdemeanor offense only when someone has it in public view, critics say that officers routinely make people they “stop and frisk” empty their pockets, then arrest them for having marijuana in public view.

The vast majority of those stopped and frisked are black or Hispanic. And under Mayor Michael R. Bloomberg, from 2002 to 2010, about 87 percent of those arrested for marijuana were black or Hispanic, while only 10 percent were white, according to a breakdown on Dr. Levine’s Web site based on data from the state Division of Criminal Justice Services.

“It is worth remembering and pointing out that U.S. government studies consistently find that young whites use marijuana at higher rates than young blacks and Latinos,” Dr. Levine said in a statement. “But the police patrols, stop and frisks, and arrest quotas are highest in black and Latino neighborhoods, and that is where the N.Y.P.D. makes most marijuana possession arrests. Mayor Bloomberg is like the Energizer bunny of marijuana arrests – he just keeps going and going and going.”

Commissioner Kelly said Wednesday that the decline in arrests after the directive was issued did not necessarily mean that the officers had been misapplying the law earlier.

“There was an allegation at a City Council hearing that I attended, that officers were telling individuals who were stopped to empty their pockets, and when they showed they had marijuana, that they were being arrested for a misdemeanor, and that’s not the intent of the law,” he told reporters. “If you have it in plain sight then it is a misdemeanor. If you’re directed by an officer to take it out of your pocket, that’s not the intent of the law. That’s what that directive was meant to address. It’s very difficult to quantify whether or not that was happening. So, the numbers are what the numbers are.”

The study tabulated arrests in which the top charge was fifth-degree marijuana possession, defined as having any amount of marijuana in public view or having 25 grams to 2 ounces. Possession of less than 25 grams of marijuana not in public view is a violation, punishable by a fine.

A bill introduced in the State Legislature last year would make having a small amount of marijuana in public view a violation, but there has been no vote on it.