FP Explainer

How Do We Know How Many Protesters Came Out in Cairo?

We don't really.

Organizers of Egypt's protests called for a million demonstrators to come to a rally at Cairo's Tahrir Square on Feb. 1, but actual estimates of crowd size have varied widely. Al Jazeera reported a crowd of "more than a million." The AP was more conservative, using "more than a quarter-million." The New York Times was vague, describing "hundreds of thousands" filing into Tahrir Square. How do they come up with these numbers?

They're probably guessing. The most widely used method of counting crowds was developed by the U.S. National Park Service in the early 1970s and relies on three factors: the size of the space where the event occurs, the amount of the space occupied by the crowd, and the density of the crowd. Crowd density can vary from "mosh pit" close -- one person for every 2 feet -- to the more common one person for every 5 feet -- your standard Tea Party rally. It's nearly impossible to get an accurate count from the ground because of an optical illusion called foreshortened perspective that makes crowds closer to the observer appear denser than they actually are, so aerial photos are generally needed.

U.S. newspapers still use a variant of this method, though the Park Service has been prohibited by Congress from releasing its own numbers since 1995, when Million Man March organizer Louis Farrakhan threatened to sue the agency for estimating the crowd size at around 400,000.

For preplanned, organized events like presidential inaugurations or the recent rallies on the Washington Mall held by Glenn Beck and Jon Stewart, news agencies contract with aerial analysis firms like the Virginia-based Digital Design and Imaging Service to get the most accurate numbers possible. Digital Design makes a computer model of the space divided into a grid. Then a balloon mounted with nine cameras is tethered over the crowd and takes photos at different altitudes and vantage points. Organizers generally try to schedule these surveys for the high point of the event, such as when Sarah Palin spoke during Beck's "Restoring Honor" rally. Digital Design then comes up with multiple numbers based on mathematical modeling, hand-counting heads within a given quadrant, and guesstimates from academics who study crowd behavior. The numbers are then averaged out. 

Estimating gets exponentially harder in a more spontaneous and chaotic event like the Tahrir Square rally. The event was called only a day before, so obviously computer models and weather balloons were out of the question. The most accurate aerial photos of the event come from satellites, which means that images can be taken only when the satellite is directly overhead and when the square is not obscured by clouds. This makes it extremely difficult to get multiple images and to judge when the crowd is at its height.

Using a simplified version of the Park Service methods, analysts at the private intelligence firm Stratfor found that even the AP's 250,000 number is pushing it. University of Illinois sociology professor Clark McPhail estimated that Tahrir Square itself could probably hold a fairly dense crowd of around 100,000.

Of course, Tahrir is essentially a giant traffic circle with multiple feeder streets, also choked with people -- and news reports Feb. 1 stated that protesters were fanning out throughout the city. Because the demonstrators were not necessarily marching in one particular direction and were pushing through vehicles, police, and other impediments, a reliable estimate for the total number of people in downtown Cairo becomes mostly a matter of guesswork.

Even using the vaguest criteria, however, Al Jazeera's estimate seems extremely generous, though that was presumably of little comfort to President Hosni Mubarak as he announced his impending departure from power.

Thanks to Curt Westergard, president of Digital Design and Imaging Service, and Clark McPhail, professor emeritus of sociology at the University of Illinois.

Chris Hondros/Getty Images

FP Explainer

Can Governments Really 'Block' Twitter?

Not really. The domain name is inaccessible, but it's not that hard to get around.

View a slide show of this week's protests in Egypt.

This week, Egypt became the latest Middle Eastern country to see massive anti-government street demonstrations. As in Tunisia earlier this month and Iran last year, activists have made heavy use of social networking sites like Twitter and Facebook -- and the Egyptian regime has responded harshly. On Jan. 25, Twitter officially confirmed reports that access to its site had been blocked. Is it really possible to do that?

Yes, but not very effectively. The Egyptian government appears to have been blocking access to the Twitter.com domain name, most likely with the assistance of the country's Internet-service monopoly TE Data. Later in the day on Tuesday, Egyptian authorities began shutting down wireless data services entirely in the areas where the protests were taking place in order to prevent demonstrators from logging on. (Facebook has also reportedly been suffering outages on Jan. 26, though the company denies that it has been blocked.) As is its habit, the Egyptian government hasn't created a redirect page for the site, but merely slowed traffic down to a crawl to give itself plausible deniability. Late in the day on Jan. 26, the site was reportedly accessible again.

Unfortunately for the censors, Twitter allows other companies to develop their own applications using its programming interface. This has led to the development of a plethora of tools that allow users to post to Twitter without ever pointing their browsers to Twitter.com. These third-party clients still appear to be functioning in Egypt. There have even been reports of activists updating Twitter through the professional résumé-sharing site LinkedIn.

It's also still not prohibitively difficult to access Twitter.com. The site has multiple IP addresses, not all of which are blocked by government censors. Savvier Egyptian web users can access one of these addresses without using the site's domain name at all. Another easy workaround is to use a virtual private network, or VPN, which fools the system into thinking you're outside Egypt.

Unlike other authoritarian states such as China or Iran, Egypt does not have a particularly extensive web-filtering operation in place. The decision to block Twitter may be a sign of how serious the regime is taking the protests, though even now the restrictions seem somewhat haphazard and arbitrary. For instance, while Bambuser, a site used to stream video to one's Facebook account from a mobile phone, has been blocked, YouTube, which has been used extensively by the protesters, is still accessible.

Some regimes have been more aggressive in counteracting the effects of social networking. During the Tunisian protests, a malicious program hosted by the country's Internet service providers was found to be stealing users' login information and passwords. In 2009, a group calling itself the Iranian Cyber Army, thought to have links to the Iranian government, hacked Twitter so that it instead displayed anti-American propaganda.

Generally, the Egyptian authorities prefer to allow opposition members to share information online so that they can closely monitor them. In some cases, they've gone as far as to ask online activists for their email and website passwords rather than shutting them down. But with riots spreading throughout the Arab world in the wake of Tunisia, the powers-that-be may have decided that blunter methods were called for.

Thanks to Mark Belinsky, co-director of Digital Democracy, and Ethan Zuckerman, senior researcher at the Berkman Center for Internet and Society at Harvard University.  

MOHAMMED ABED/AFP/Getty Images