China’s Q1 2020 GDP drop was -19.4%, not the officially reported -6.8%

The veracity of China’s economic data is an ongoing topic.

Here’s a paper from four researchers at universities in the US and … China.
Their paper, based on traffic flow data, finds that GDP in Q1 of 2020, when China imposed severe restrictions in response to the COVID-19 outbreak, declined by -19.4%, not by the official figure of -6.8%.