Real and fake data in Shanghai's informal rental housing market: Groundtruthing data scraped from the internet

China's planned mega-cities contain hidden, informal housing markets. We analyse Shanghai's 'group rental' market in which formal commercial and residential units have been illegally converted into extremely crowded dormitories. In 2016, we collected more than 33,000 online classified advertisements for beds in group rental apartments and find that this market serves a specific demographic with robust preference order patterns. Furthermore, groundtruthing fieldwork revealed that the scraped online data misrepresented the market. Therefore, we also collected a second set of 'real' market data for comparative analysis. The study highlights both the exciting possibilities and the limitations of using online content to study informality.