Why don’t I know anyone personally who died in the Sept. 11th disasters? ‘The World Trade Center question falls into a category that mathematicians call “small-world” or “degrees of separation” problems. The key work in the field for our purpose is being done by a team of five social scientists and mathematicians who have already written a paper on 9/11, “Estimating the Ripple Effect of a Disaster.” It is being published in the journal Connections. You can read it here…’ The upshot of the back-of-the-envelope calculation is that around 1:200 Americans is likely to have known a victim. “The math offers a kind of solace, too. Yes, it shows that we are less directly connected than we think to 9/11. But it also shows that we’re more indirectly connected. The most amazing statistic of Bernard et al. is how many people knew someone who knew a victim. According to their estimates, essentially all Americans — more than 80 percent of them — know someone who knows someone. We are all mourners at the second degree.” Slate. Jargon Watch: While we’re at it, here are the results of a Google search on “back-of-the-envelope.”