Friday, September 17, 2021

# How many marched in Hong Kong? The maths of measuring crowds

## With estimates ranging between 3.8 lakh and 2 million, a look at various methods that are used, or can be used, for assessing the size of a crowd.

In this Sunday, June 16, 2019, photo, tens of thousands of protesters march on a street to protest against the unpopular extradition bill in Hong Kong. (AP Photo: Kin Cheung)

On June 12 and June 16, huge crowds took to the streets in Hong Kong, protesting against an extradition law. How huge is huge? With estimates ranging between 3.8 lakh and 2 million, a look at various methods that are used, or can be used, for assessing the size of a crowd.

2 million

This is the estimate of the Civil Human Rights Front, which organised the June 16 protest, according to Reuters. The news agency quoted a member of the Front as saying that they simply counted everyone.

What does it take to count 2 million — how many people counting other people, and for how long? In a report on crowd measurement, Reuters made its own estimate. If protesters were counted at the rate of 10 people per second, counting 2 million people would take 552 man-hours of work. This protest lasted less than 8 hours, Reuters noted.

3.80 lakh

In flow method, take a narrow stretch, measure the number of people passing through it in a given time, then multiply by total time for crowd to pass. (Adapted from a Reuters graphic)

The Hong Kong Police Force’s estimate. They told Reuters they measured how many people were in the designated protest area during the “peak” — the time when the crowds were most dense.

Jacobs’ method

Named after Herbert Jacobs (1903-87), a professor of journalism at the University of California-Berkeley, this method works best for a rally in one place. Take the average number of people per square metre (or square foot), and multiply by the protest area in square metres (or square feet). Jacobs assumed that a loose crowd has one person every 10 sq ft, and a very dense crowd has one person per 2.5 sq ft. However, the method is not always flawless. In a paper on the subject, Penn State University researchers observed that people do not uniformly distribute over a space, but clump together into groups or clusters.

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Flow method

For a protest on the move, measure the number of people passing through a given stretch over a short period, and multiply it by the duration of the event. Ideally the narrowest stretch is chosen.

This method assumes that no one is joining the march after the narrowest point. For this reason, using a location close to the end to measure flow is ideal, Reuters explained in a graphic and write-up on the subject.