The authors are graphics editors for The Times.

The authors are graphics editors for The Times.

The criminal justice reform bill known as the First Step Act, which President Trump signed into law on Friday, has been lauded as a sorely needed instance of bipartisan lawmaking. The law will reduce sentences for federal prisoners while seeking to balance public safety needs.

It is a bright moment in a highly partisan time. The title claims it is the “first step” of further reform, but this legislation follows a growing number of state-level moves that are gradually undoing decades of tough-on-crime policies that caused the nation’s prison population to swell.

Here, you can check your knowledge of the American criminal justice system and how the First Step Act fits in.

Who is affected?

In the United States, people are locked up in correctional facilities under the jurisdiction of federal, state and local authorities. The First Step Act applies only to some federal prisoners.

As of 2016, the share of people incarcerated in the federal system was…

Show me the answer … less than a tenth of the total number incarcerated in the country. That leaves more than 1.9 million incarcerated people who will not be affected by the law.

Among other provisions, the law retroactively reduces sentences for crack cocaine, shortens mandatory minimums for some offenses and makes it easier for prisoners to earn early release. Also included are affirmations of policies that place prisoners near their families or homes and prohibit the shackling of pregnant inmates.

These reforms will make a difference, albeit for a relatively small number of federal prisoners. Some advocates think this federal law will send a signal to states that have lagged in enacting criminal justice reform.

Why does the system need reform?

Compared with other countries’, the U.S. incarceration rate is …

Show me the answer … the highest in the world. In second place is El Salvador, which has the world’s highest rate of intentional homicide, according to the United Nations.

A recent study from Cornell found that almost half of American adults have had a family member jailed, the result of a criminal justice system that is impractical, unjust and ineffective. State correctional budgets grew to nearly $50 billion in 2015. Black men receive harsher sentences than white men for the same crimes. And research suggests that increased incarceration has little to no effect on preventing crime.

How much crime is there now?

The incarceration rate has increased by 50 percent since 1990.

1,000 inmates per 100,000 residents 800 Rate peaked in 2007 600 400 450 inmates per 100,000 residents 200 117 0 1960 1970 1980 1990 2000 2010 2017 1,000 inmates per 100,000 residents 800 Rate peaked in 2007 600 400 450 inmates per 100,000 residents 200 117 0 '60 '70 '80 '90 '00 '10 '17

Simultaneously, the violent crime rate has …

Show me the answer … nearly halved.

The disconnect between public perception of crime and what is actually happening is part of the reason criminal justice reform has been difficult to pass.

How has the prison population changed?

The incarcerated population has grown sevenfold since 1970, far faster than the country’s population.

Between 1986 and 2013, the average number of months served in federal prisons …

Show me the answer … more than doubled.

As of 2016, one out of every nine prisoners in the country was serving a life sentence, a number that has increased nearly fivefold since 1984. Because of long sentences, the average age of those incarcerated has risen in tandem with the average health care cost per inmate. But older people are less likely to commit another crime when released.

In recent years, states across the political spectrum have made concerted efforts to safely reduce correctional populations, said Jake Horowitz, the director of Pew’s public safety performance project.

Whether reforms should apply retroactively and to people convicted of violent crimes has been a point of contention. An earlier version of the First Step Act would have retroactively shortened some lengthy sentences, but the clause was removed to win the support of law enforcement groups and President Trump.

To reduce the number of people serving lengthy prison terms, more retroactive sentence reductions will be necessary. Because people convicted of violent offenses are concentrated in state prisons, states will need to make policy decisions that take this difference into account, such as more funding for parole and re-entry programs that help people transition back into society and reduce recidivism.

New York was able to reduce its correctional population over the past two decades without suffering an increase in crime by changing policing practices and emphasizing treatment programs, community service and fines instead of prison time. Other states have enacted reforms that include retroactive sentence reductions, legalization of marijuana and improved risk assessment for pretrial detention and parole decisions.

“Crime and incarceration are not a zero-sum game,” Mr. Horowitz said. “You can actually have less of both.”