Counting heads

It’s not all porto and pastéis de nata for me here in Portugal. I spent yesterday doing journalism.

I was contacted a couple of days earlier by Curt Westergard, whose company uses cameras on tethered balloons to produce high-resolution aerial photos. He had been hired by CBS News to get images of the crowd that gathered Saturday for the Glenn Beck “Restoring Honor” Tea Party rally at the Lincoln Memorial in Washington. CBS also wanted a credible estimate of the size of the crowd. That’s where I came in.

I started doing crowd estimates back in my Miami Herald days, for local events such as a visit by Pope John Paul II or the annual Calle Ocho street festival. After the 2008 presidential election, I wrote a piece about crowd counting for, and that led to news media requests for an estimate of the crowd for President Obama’s inauguration. I used a satellite image taken over the National Mall on January 20, 2009, to estimate the crowd there at about 800,000.

Crowd counting, particularly of political events, always is controversial. The organizers of the event inevitably hype their crowd estimate — often grossly — to demonstrate the popularity of their cause, and opponents inevitably underestimate to fit their own agenda. Because of the wild pre-inauguration predictions of how many would attend in person — up to 5 million! — my reality-based estimate was ignored by many left-wing commentators and embraced by those on the right. shot for CBS News

Naturally, I expected more of the same about my Beck rally estimate. To calculate it, I used’s very striking images (MUCH larger than the thumbnail posted here) to make density estimates across different zones of the crowd; a variety of ground-level images from news photographers and attendees who posted their photos on Flickr; and Google Earth to measure the square footage of the different zones. Yes, I included the crowd areas under the trees; the full-size Airphotolive images were detailed enough to discern the edges of the crowd even there.

My estimate is that about 80,000 people were at the rally. Ryan Shuler, an Airphotoslive image analyst, used the same images and a different grid-density method to produce an estimate of 87,000. Considering the error margins around our separately-calculated estimates, they are statistically identical. CBS went with the 87,000 figure, which I certainly can accept.

Now the fun begins in the blogosphere. NBC News, the New York Times, and other large media outlets that didn’t attempt a scientific estimate uncritically accepted Beck’s claim of “300,000 to 500,000”. (At least Rep. Michelle Bachmann’s laughable claim of 1 million isn’t being treated seriously.)

The CBS News estimate immediately was vilified by conservative bloggers, and often rabidly-virulent comments from readers are being posted on news stories that mention the CBS estimate. I won’t post more links, but you can find plenty by Googling “beck rally attendance” and similar search terms.

The frothing underscores the problem with hyped predictions of crowd size. Organizers and supporters are forced to insist loudly that the actual crowd met or exceeded their expectations, for fear that the realistic estimate will be painted as a disappointment. The time-honored way to dismiss scientific estimates that don’t reflect the pre-event hype is to claim political bias on the part of those doing the estimate. I am amused to see that those who embraced my Obama inauguration estimate as soberly realistic are now attacking the Beck rally estimate, produced using exactly the same methods, as deliberately biased.

I expect that kind of behavior from partisans on both sides. I am disappointed, though, by the many responsible news organizations that failed to produce their own independent estimates and instead reported only ungrounded hype. Their readers and viewers deserve better journalism than that.

[DOIG afternote: I welcome reasoned comment and questions. Don’t bother sending ideological rants from any direction; I trash those. Also, it may take a while for your comment to be read and approved. I’m in Portugal five hours ahead of DC, also the flood of hits on the site has slowed it way down.]