It’s common to have a conversation about criminal justice reform that includes an anxious reference to the “mass incarceration crisis.” But as legal academic John Pfaff points out in his new book “Locked In,” the phrase might actually be meaningless because we don’t know the right number of offenders to incarcerate. For instance, there are 43,000 people occupying Michigan prisons right now. Is that “mass incarceration?” If so, what is “normal incarceration” and how do we get there? There’s no clear answer to these questions, but here’s how to approach the issue.

First, determining the “right number of people” is influenced by your point of view on more fundamental questions. If you’re a prison abolitionist who believes prisons are innately corrupting and harmful to society, your right number is practically to zero. But if you’re a tough-on-crime proponent who sees long sentences as an effective means to deter criminal activity, your number would be much larger.

Second, while reformers across the spectrum tend to agree that the incarceration rate is too high, they don’t agree on how to scale it back. And here is where the complexities of calculating the right number of offenders to incarcerate start to appear. Dozens of variables contribute to the size of the prison population, and each is quite difficult to measure.

Take the idea of justice, for example. The state punishes offenders on behalf of their victims, but it’s not always clear what sentence will be appropriate. If a store is robbed, is justice served for the store owner if the offender is given a two-year sentence? A five-year sentence? Compounding the problem is the fact that plea bargaining increases the uncertainty over how long — or even whether — a given offender will serve time. Is justice served when pleas are bargained down?

Uncertainty persists at every level of the criminal justice system. Once the offender enters prison, incarcerating him averts any crimes he might have otherwise committed. But we should always ask: Is this the most effective way to improve public safety, or are there better alternatives? We have no way of counting the averted crimes, and some research suggests that incarcerating people for too long may induce them to commit crimes later that they otherwise might not have committed.

Decisions about when and how a person can be released from prison are also fraught with complexity. Michigan’s truth-in-sentencing policy means that a prisoner will serve every day of the minimum sentence imposed by the judge at trial. But after that, it’s up to the parole board to decide whether and when to release a prisoner before he has served the maximum statutory punishment. What factors should the board take into account? Is there some point after which keeping a prisoner incarcerated will do more long-term harm than good? Should we expect imprisonment to reform convicts or just mete out punishment?

Finally, budgetary concerns often affect all these questions. If the price tag gets too high, the question must shift from the right number of offenders to incarcerate to the right type of offender to release. That’s a different question, requiring a different analysis. The state only has so much money, and legislators must consider not only the criminal justice system but many other priorities when they create budgets.

All these factors and more influence the size of the prison population and they should be accounted for as we discuss ways to determine the optimal number of prisoners. There are many questions about how well and when prison works and considering them in light of budget constraints is a difficult task. It’s important to think critically, not only about the size of the prison population but also about ways to improve the criminal justice system.

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