For years, we’ve known that the jail incarceration rate in the United States is higher than the global average, and that incarceration inflicts enormous financial and social costs on individuals, families, and communities.

But we know less about one of the biggest contributors to jail incarceration, which is who is being arrested and why. Apprehension by the police is often the first point of contact people have with the criminal justice system, and this interaction can lead to incarceration. By understanding trends in police arrests across time, location, offense type, and arrestee demographics, we can take an important step forward in ameliorating over-criminalization and mass incarceration.

To understand police arrest trends in the United States, we rely on data reported to the FBI by police agencies. Until the recent launch of the Vera Institute of Justice’s (Vera’s) Arrest Trends data tool, this data had been difficult to access and interpret, especially for people without extensive research training. With the development of Arrest Trends, however, people can now easily examine and analyze decades of police data from our nation’s nearly 18,000 police departments both at the local and national levels. In this way, Arrest Trends is a valuable and much needed tool that powerfully illustrates the scale of arrests in the United States.

However, because approximately 30 percent of police agencies don’t report any of their arrest data to the FBI, our understanding of arrest trends is incomplete. And, because arrest trends shape our understanding of crime rates—and, consequently, whether police practices are effective—an incomplete picture of arrests is a barrier to tracking progress and devising solutions for police reform.

Vera’s recent analyses—using data from Arrest Trends—reveals that failure to report policing data to the FBI may not occur at random. In fact, the likelihood of a given agency reporting arrest data to the FBI changes based on community racial composition. More specifically, agencies in counties with larger populations of Black people and Native American people report fewer months of arrest data to the FBI on average than agencies in counties with larger populations of white people. Because studies have consistently shown that Black people and Native American people are more likely to live in poverty than white people, one potential explanation for these results is that counties with larger proportions of people of color and concentrated poverty house agencies with fewer resources, making reporting more burdensome.

Regardless of whether these differences in reporting are because of race or other factors—such as socioeconomic status of the community—this trend is concerning. Based on the communities that report their arrest data to the FBI, we know that communities with larger populations of people of color and communities with higher poverty levels experience disproportionate levels of arrests. Other research shows us that these communities often experience more police contact, poorer perceptions of the police, and higher incarceration rates. Given these trends, it is likely that the agencies that don’t report their data to the FBI are serving communities that experience negative contact with the criminal justice system and are arguably in need of attention and resources to advance policing practices.

But without complete arrest data being reported to the FBI, a full picture of who is being arrested, for what offenses, and to what extent remains unknown. As a result, this missing data prevents advocates, researchers, and politicians from recognizing and addressing police practices that may contribute to negative police­-community relations and higher incarceration rates.

Only when agencies make all of their data available can we begin to fully understand the landscape of arrests, create alternatives to enforcement, and ensure accountability in police practices. By publicly displaying information about the lack of data, Vera’s Arrest Trends initiates the dialogue about the importance of data completeness and empowers community members to explore whether their own agencies report data to the FBI. In other words, Arrest Trends provides community members with the access to information necessary to advocate for transparency in arrest practices locally and nationally.