TOPSHOTS Demonstrators shout slogans during a march in St. Louis, Missouri, on November 23, 2014 to protest the death of 18-year-old Michael Brown. (Jewel Samad/AFP/Getty Images)

THE LEADERS of a protest movement against excessive police force are weighing their next steps, The Post’s Wesley Lowery reported this week. The assessment follows both greater success than anticipated in triggering rallies across the nation and some backlash that blamed the protests (baselessly, in our view) for last weekend’s slayings of two New York City police officers .

As the leaders consider their next moves, we hope they do not lose sight of an early achievement, which will require attention to bring it to fruition. One test of any social movement is its power to inspire legislation. By that measure, the protests that followed the deaths of Michael Brown and Eric Garner already demonstrated strength. Before adjourning, Congress adopted and sent to President Obama a significant bill that could help provide a crucial missing ingredient for reform: accurate information.

The Death in Custody Reporting Act would give state and local law enforcement agencies incentives to report to the Justice Department all deaths of people, for any cause, while they are under arrest, in the process of being arrested, detained or incarcerated. Agencies that want to retain federal funding would have to fill out a brief form for each case, including the name, age, gender and race of the deceased, along with a short explanation of the circumstances. While the statute would cover nonviolent deaths, and even deaths from illness, as well as violent ones, the main hope is that it will enable data-crunchers to analyze patterns in the use of force and thereby spot potentially unjustifiable trends at particular departments.

To be sure, the measure is not exactly brand-new. A version passed in 2000 but expired in 2006; in only three of those years did the Justice Department’s Bureau of Justice Statistics have authority to require quarterly reports — a requirement with which compliance was less than total. Since then, voluntary reporting has continued but also has proven spotty. A recent investigation by the Wall Street Journal found that nearly 45 percent of the justifiable homicides tallied by the nation’s 105 largest police departments went unreported to the FBI between 2007 and 2012. Florida and New York, two of the largest states in the country, accounted for 290 of 580 missing cases analyzed by the paper.

In the wake of the Brown and Garner tragedies, there is reason to be optimistic that this latest iteration of the bill will prove more effective and durable. Right now, a big part of the problem is that no one in the debate over police conduct really even knows what they don’t know. “Hopefully when we get information we can figure out how to reduce the numbers of people dying,” said Rep. Bobby Scott (D-Va.), who shepherded the bill through the Republican-dominated House.

We can’t think of a better reason to look forward to Mr. Obama’s signature on the measure — and to keep pressure on the Justice Department to ensure conscientious implementation of it thereafter.