Cumbria and the risk from guns
As the prime minister and home secretary meet people in Cumbria today there will be voices urging them to do something to ensure that the horrors of this week can never be visited on another community.
David Cameron has already warned against a knee-jerk response but there will be a review of our gun laws when the facts are known, the dust has settled and the scars have begun to heal.
Before ministers consider tightening what are already some of the toughest gun-ownership laws in the world, however, it might be worth seeking answers to two questions:
• Is there a link between gun ownership and mass killings?
• Would further restrictions on gun ownership be justified?
To answer the first question, I have done a few back-of-an-envelope calculations and pulled together all the documented cases of mass shootings I could find from the last 50 years.
Only 18 countries have experienced more than one such tragedy.
Column two shows that the USA has experienced the greatest number with 24 cases, followed by China with 18 and Israel with 11. Given that Israel has a population of around 7.5 million compared with China's population of 1,300 million (see column three), it is clearly important to see such incidents in the context of the size of the country.
Column four provides a rough estimate of the relationship between population and mass killings - the number of incidents in the last half a century for every 10 million people.
The last column lists the number of legally held guns per 100 people. The main source for this is the Small Arms Survey [2.02MB PDF] compiled by the Geneva Graduate Institute of International Studies.
The experience of Israel and the Palestinian Territories is by far the most acute when it comes to an individual opening fire on a crowd of strangers.
It could be argued that these events, in the context of the crisis in the Middle East, are fundamentally different from, say, school shootings in the United States or this week's massacre in Cumbria.
It is wrong to argue that the only factor increasing the risk from mass killings is the availability of guns; on the other hand, without access to firearms, such slaughters would be almost impossible. Derrick Bird could not have completed his murderous journey without his guns.
I have not been able to find figures for gun ownership in Israel but it is widely reported to be very high. For the purposes of this exercise, however, let us put the Middle East situation to one side and focus on the countries for which we do have ownership figures.
The seven countries with the highest known levels are USA, Yemen, Finland, France, Canada, Germany and South Africa. Britain, it emerges, has relatively low levels of gun ownership by international standards, estimated at 5.6 guns per 100 people.
Excluding those places for which we don't have ownership data, the seven countries with the highest number of mass killings per capita are exactly the same seven countries - albeit in a different order.
This suggests a correlation between access to guns and the risk of suffering mass shootings. Hardly surprising, perhaps.
It is worth pointing out that there are some countries with gun-ownership levels above 30 in 100 which have not experienced two or more mass killings in the last 50 years, Sweden, Switzerland and Serbia.
However, there was a mass killing in Switzerland in 2001 in which 14 people died, in Sweden in 1994 when seven people were shot dead and nine people died in a mass shooting in Serbia in 2007.
The second question, then, is whether the events in Cumbria justify further restrictions on gun ownership.
What the table shows is just how rare these kinds of incidents are. Britain has only experienced three in the last 50 years - possibly ever: Hungerford in 1987, Dunblane in 1996 and now Cumbria.
All are profoundly shocking and tragic moments in our history. All must prompt us to look at how our government and our society can respond most effectively. But there is always a balance to be struck between reducing small risks and restricting vital freedoms.
Update 1459: Thank you to Steve Hunt (comment 6 below) for pointing out that South Africa should move further up the table - corrected above.