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Perhaps you've seen the headlines over the last few years trumpeting the slow, but significant decline in U.S. prison populations.

"State, Federal Prison Populations Decline Simultaneously for First Time in 36 Years," announced the Pew Charitable Trusts last September. "Federal Prison Population Declines for First Time in Decades," proclaimed NBC News in 2014. "The prison population in the United States dropped in 2012 for the third consecutive year," reported the New York Times in 2013.

The fact that there are fewer people living in prison is, of course, good news. But is it the whole story? Do these reported drops in prison populations paint a complete picture of the United States' complicated and controversial penal system?

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Not according to John Pfaff.

Pfaff is a professor of law at Fordham University. There, his research "focuses primarily on empirical matters related to criminal justice, especially criminal sentencing." He is, in other words, an expert when it comes to the ins and outs of our nation's prisons. And, as it happens, it is the "ins" and "outs"—specifically, the numbers of people actually going into and out of prison—that caught Professor Pfaff's attention on Tuesday morning.

It comes down, explains Professor Pfaff over the course of 24 tweets, to a question of metrics. While we trumpet announcements about declining prison populations, we do so while glossing over the decidedly less-rosy stats on prison admittance.

Of course, things can vary, state-by-state.

And things are still better than they used to be.

For those out there who have a hard time wrapping your brains around the difference being explored here, Professor Pfaff offers a simple example:

In other words, while prison populations may be down, that doesn't necessarily mean there's been a decrease in the sheer number of people being affected by prison terms.

The key, explains Professor Pfaff, isn't simply prison reform, but the right kind of prison reform.

Ultimately, then, it comes down to really taking the time to examine the data, rather than accept a convenient narrative.

And just in case you thought he forgot