Exactly how many people took part in Hong Kong's massive pro-democracy rally on Tuesday? It depends on who you ask. In Hong Kong's culture of constant protest, vast differences in turnout estimates are common. But this year, amid heightened political sensitivities, the number is being debated with particular heat.

Rally organizers estimated that 510,000 took part in the annual July 1 march, while the Hong Kong police's official estimate was a more modest 98,000. Two independent analyses placed turnout between 140,000 on the low end and 172,000 on the high end.

As it turns out, the numbers differ in part due to the fact that they were counting different things. The police, for example, didn't try to measure the total number of participants—instead, they focused on counting the maximum number of marchers at any given moment. By contrast, the protest's organizers say they included those who left early or joined the march through side streets along the four-kilometer route.

The estimates were also conducted with varying statistical rigor. Rally organizers obtained their figure through three teams of volunteers performing a manual count at various points along the route. To include the protesters that took part on side streets, organizers simply multiplied their original count by 1.5.

The University of Hong Kong, which provided an independent estimate, put turnout at between 154,000 and 172,000 people. Researchers led by Dr. Robert Chung, director of the Hong Kong University Public Opinion Program, manually counted passers-by at a checkpoint and subsequently conducted a random telephone survey of Hong Kong residents to check their count.

Local English-language daily South China Morning Post, which also produced an independent estimate, was the only one that used computer analysis in a significant way. To do so, the paper measured protester density by analyzing aerial photos from three locations along the rally route, ultimately concluding that 140,408 people participated in the march.

"The advantage is that it's the only method based on area," said Thomas Lee, an analyst commissioned by the SCMP, in a video explaining his methodology. "The formula we used is very simple. It's based on area multiplied by density." His methodology is similar to the Herbert Jacobs method, a common crowd-counting system using area and density to estimate the size of public gatherings.

While both independent estimates may perhaps appear more reliable than either that of the police or protesters, neither HKU nor the SCMP counted participants who entered the protest mid-way or left before reaching the march's end. That means that both likely undercounted the number involved. As protesters waited for the march to begin on Tuesday afternoon, rally organizers explicitly encouraged those gathered at the start line to tell their friends not to cram into Victoria Park, which was at full capacity, and instead join the rally in progress.

Turnout particularly matters because this year's protest is widely seen as a gauge of Hong Kong peoples' satisfaction with the political regime in Beijing and their hand-picked chief executive, Leung Chun-ying. In an indication of how closely Beijing is watching Hong Kong, for the second time in three days, the Communist Party mouthpiece People's Daily ran a front-page editorial about Hong Kong's relationship with mainland China. "There are also people worried about whether the central government will narrow Hong Kong's high degree of autonomy," it wrote, referring to the "one country, two systems" principle that governs the former British colony's relationship with mainland China. "This is baseless," it wrote. "The central government's fundamental policy on Hong Kong has not changed, and never will change."

The quibbles over participation may be irrelevant. Judging from the tone of the People's Daily, Beijing is feeling the heat regardless of whether 100,000 or half a million took part.

-- Edward Ngai

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