In one sense, searching for these statistical gaps is like fishing blindfolded—how can someone know what they don’t know? But some absences are more obvious than others. Bruce Western, a professor of sociology at Harvard University, cited two major gaps. One is the racial demography of arrests and criminal records. An estimated 65 million Americans, or roughly 25 percent of the U.S. population, have a criminal record of some kind. But the racial makeup of those records isn’t fully known. “There are estimates, but with [65 million] people in the FBI criminal record database, we have no systematic knowledge of their demographics,” Western told me.

There may be many missing statistics from the realm of policing, but even greater gaps lie elsewhere. Thanks to the FBI’s Uniform Crime Reports, police departments might actually be one of the better quantified parts of the criminal-justice system. Prisons also provide a wealth of statistics, which researchers have used to help frame mass incarceration in its historical and demographic content. The Justice Department’s Bureau of Justice Statistics maintains an annual report on the size of the U.S. prison population. The report includes state-by-state demographic statistics like inmate ages, races, crimes committed, and other crucial data for researchers and policymakers.

But while current prison statistics give a good sense of the size and scale of mass incarceration, they provide little information on conditions inside the vast constellation of American prisons. Perhaps the most glaring gap is solitary confinement. No one knows exactly how many people are currently kept in isolation in American prisons. “There are estimates, but no official count nationwide,” Western said.

The most commonly cited figure comes from the Vera Institute for Justice, which estimates that around 80,000 people are in solitary confinement at any given time in the United States. But precise statistics remain elusive despite the substantial policy implications. There is abundant evidence that prolonged isolation can be mentally and psychologically traumatic. Justice Anthony Kennedy told Congress in April that solitary confinement “literally drives men mad.”

For data to be relevant, it must be gathered broadly and repeatedly. 80,000 people is a significant number, but even the total is accurate, it’s analytically useless without context. For example, does this figure represent a decline or rise in solitary confinement compared to five years ago? How does it vary from state to state? What are the racial, gender, and age breakdowns of those 80,000 people? For how long are they kept in isolation, and why?

Another major gap in prison statistics is the number of non-sexual assaults behind bars. Although Congress mandated the collection of sexual assault statistics with the Prison Rape Elimination Act in 2002, prisons are not required to report ordinary assaults to the Bureau of Justice Statistics. “We have anecdotal evidence and some data in individual states that violence is rising in prisons, but we don’t have good, national database collection on that,” said Marie Gottschalk, a professor of political science at the University of Pennsylvania.