In the wake of the massacre at Sandy Hook Elementary School in 2012, the National Rifle Association put together a panel of experts to come up with recommendations for addressing such incidents in the future. The result was a document that ran hundreds of pages and focused, predictably, on hardening schools against the threat of armed intruders.

It recommended a single point of entry to a school’s grounds with fencing preventing access elsewhere. Shrubs should be kept away from the buildings and trees planted at a distance, to prevent an assailant from climbing to gain entry via upper windows or the roof. Specific entryway schematics were provided to maximize sight lines and minimize the ability of a shooter to get past without confrontation. Ballistic glass was recommended for interior windows and steel plating for entry-point desks.

At the end was pre-written sample legislation to allow teachers to carry guns.

In his speech to the Conservative Political Action Conference on Thursday, NRA head Wayne LaPierre repeated his call for strengthening security in America’s schools.

“In every community in America, school districts, PTAs, teachers unions, local law enforcement, moms and dads,” LaPierre said, “they all must come together to implement the very best strategy to harden their schools, including effective, trained, armed security that will absolutely protect every innocent child in this country. And that has to happen now.”

The NRA is in a tough position on school shootings. It’s fundamentally opposed to restrictions on weapons like the one that 19-year-old Nikolas Cruz purchased and allegedly used to kill 17 people at Marjory Stoneman Douglas High School in Parkland, Fla., last week. But in the face of stiff public opposition, it has to propose some solution. So it does: Massive infrastructural investments in the nation’s thousands of schools that might help to keep the death toll down in the event of an attack.

It’s worth noting, both in light of last week’s massacre and the NRA’s response, that the odds of a shooting at a school in the United States are still very, very low.

The pro-gun-control group Everytown for Gun Safety compiles data on firearm discharges at schools, including accidental firings, attempted suicides and attacks such as the one in Parkland. Since the beginning of 2013 — since Sandy Hook, in other words — it has tallied 290 such incidents. A bit more than half of those are categorized as “attack on other persons(s) resulting in injury or death,” the sort of assault we saw in Parkland. (Although  these incidents are not all exactly like Parkland. Not all involved students hoping to kill as many people as possible; some involved adults targeting specific individuals, for example.)

Broken down by state and type (with “incidents targeting other people” broken out from the total), there’s an expected link between how populous a state is and how many incidents it has. That said, states such as New York, California and Illinois have fewer incidents per capita than Florida, Georgia and Texas. In several states, there were no incidents at all.

But we’re interested in the likelihood of such an incident at a given school. Using data from the Department of Education, we tally up public elementary, middle and high schools in the United States by state.

This allows us to do a calculation.

In Alabama, for example, there are 1,519 schools (or there were in the 2015-2016 school year). There were 10 shooting incidents in those schools from 2013 through February 2018, six of which were attacks on other people. Assuming a 180-day school year, that’s about 930 days that the schools were open since January 2013. (Some of the shooting incidents happened after school hours, but we’ll ignore that for now.)

In other words, there were about 1.4 million school days in Alabama since 2013 — that is, 1.4 million units in which one school was open one day. And on 10 of those units — a day at a school — a shot was (or shots were) fired.

So once every 140,000 school days or so, there was a shooting incident at an Alabama school. If you apply that proportion to one of Alabama’s 1,519 schools, that means you would expect a shooting incident at that school once every 782 school years. If you’re looking only at the six firearm attacks, it’s once every 1,304 school years.

How does that work, given that there were 10 attacks in Alabama since 2013? Well, because there are 1,519 schools, and it has been a bit over five years.

This ignores a lot of other factors that might come into play. But it gives a sense of the rarity of a shooting incident at any particular school in any given state.

Below, the same calculation for each state (and nationally). Shootings are actually relatively common in Alabama, despite how rarely one might be expected at an individual school. They are much less common in New York.

We’re only looking at public schools here, mind you, so these figures are actually lower than they might otherwise be.

The point is that investing millions or billions of dollars to “harden” the nation’s 100,000-plus schools is a lot of money spent to prevent something that happens only very rarely, even though each time it happens is a tragedy.

This argument might easily be extended, too, to question the need for curtailing firearm ownership if school shootings are so rare. It’s a fair point. It’s worth noting, though, that while school shootings only happen at schools, schools aren’t the only place shootings occur.