Detroit is the largest American city to officially challenge its 2020 Census count. One of its pieces of evidence is a survey done by the Detroit Metroopolitan Ares Communities Study supporting its claims that the city’s population was undercounted.
Professor of public policy and sociology Jeffrey Morenoff told Michigan Radio's Stateside program, that Detroit had one of the lowest online Census response rates of any major city, and so “needed a lot more non-response follow up, as they call it in the Census...Which means teams on the ground going around to neighborhoods, knocking on doors and trying to see where people are living.” But he noted “that work got started late in Detroit, and perhaps was under-resourced in Detroit.”
He also told the Associated Press that " A drop of 31,000 residents would be really implausible.”