The following are facts compiled by Ezra Klein of the Washington Post

What follows here isn't a policy agenda. It's simply a set of facts - many of which complicate a search for easy answers - that should inform the discussion that America desperately needs to have.

1. Shooting sprees are not rare in the US. Mother Jones (Website) has tracked and mapped every shooting spree in the past three decades. ''Since 1982, there have been at least 61 mass murders carried out with firearms across the country, with the killings unfolding in 30 states from Massachusetts to Hawaii,'' they found. And in most cases, the killers had obtained their weapons legally.

2. Eleven of the 20 worst mass shootings in the past 50 years happened in the US. In second place is Finland, with two entries.

3. Lots of guns don't necessarily mean lots of shootings, as you can see in Israel and Switzerland. As David Lamp from the Cato Institute writes, ''In Israel and Switzerland, for example, a licence to possess guns is available on demand to every law-abiding adult, and guns are easily obtainable in both nations. Both countries also allow widespread carrying of concealed firearms, and yet, admits Dr Arthur Kellerman, one of the foremost medical advocates of gun control, Switzerland and Israel 'have rates of homicide that are low despite rates of home firearm ownership that are at least as high as those in the United States'.''

4. Of the 11 deadliest shootings in the US, five have happened since 2006. That doesn't include the Newtown, Connecticut, shooting.

5. America is an unusually violent country. But it's not as violent as it used to be. Kieran Healy, a sociologist at Duke University, in July made a graph of ''deaths due to assault'' in the US and other developed countries. The US is a clear outlier, with rates well above other countries. As Healy writes, ''The most striking features of the data are (1) how much more violent the US is than other OECD countries … and (2) the degree of change - and recently, decline - there has been in the US.''

6. Gun ownership in the US is declining. ''For all the attention given to America's culture of guns, ownership of firearms is at or near all-time lows,'' political scientist Patrick Egan, of New York University, wrote in July. ''Long-term trends suggest that we are in fact currently experiencing a waning culture of guns and violence in the US.''

7. More guns tend to mean more homicide. The Harvard Injury Control Research Centre assessed the literature on guns and homicide and found there's substantial evidence that indicates more guns means more murders. This holds true whether you're looking at different countries or different states.

8. States with stricter gun control laws have fewer deaths from gun-related violence. Last year, economist Richard Florida dived deep into the correlations between gun deaths and other kinds of social indicators. Some of what he found was, perhaps, unexpected: higher populations, more stress, more immigrants and more mental illness were not correlated with more deaths from gun violence. But one thing he found was, perhaps, perfectly predictable: states with tighter gun control laws appear to have fewer gun-related deaths.

9. Gun control, in general, has not been politically popular in the US. Since 1990, Gallup has been asking Americans whether they think gun control laws should be stricter. The answer, increasingly, is that they don't. ''The percentage in favour of making the laws governing the sale of firearms 'more strict' fell from 78 per cent in 1990 to 62 per cent in 1995, and 51 per cent in 2007,'' Gallup reported after a mass shooting in Tucson, Arizona, last year. ''In the most recent reading, Gallup in 2010 found 44 per cent in favour of stricter laws. In fact, in 2009 and again last year, the slight majority said gun laws should either remain the same or be made less strict.''

10. But particular policies to control guns often are. An August CNN poll asked Americans whether they favour or oppose a number of specific policies to restrict gun ownership. And when you drill down to that level, many policies, including banning the manufacture and possession of semi-automatic rifles, are popular. About 90 per cent support background checks and no guns for felons or the mentally ill.

11. Shootings don't tend to substantially affect the views of Americans on gun control. That, at least, is what the Pew Research Centre found in a poll taken after the Colorado movie theatre shooting in July that killed 12.