The simple metaphor of a bathtub is widely used to capture these dynamic relations within human migration. The level of population in a city is like the level of water in a bathtub, held steady by an inflow from the faucet and an equal outflow from the drain. A rising water level could be evidence of faster inflow, or it might be that the drain has been clogged. Similarly, a falling water level might be attributed to an inflow made slower, signifying a weakened preference by human movers to arrive, but it just as well could be due to a more open drain. Demographer Ken Johnson is the most prominent to have voiced the view that postrecession population gains in cities are due to a clogged outflow rather than a stronger inflow (Johnson, Winkler, & Rogers, 2013).
In urban affairs, far more attention is paid to the inflows because they are much more visible and researchable. Those newcomers are present to be interviewed, whereas the departed outflow has disappeared to other, unknown destinations. Thus, for these reasons, many times urban analysis is subject to inflow bias: our interpretations are blind to the equal effects of the unobserved outflow. However, if we survey today’s residents about their future intentions, we might learn about their intended outflow, and yet we cannot ask future incoming residents about their intentions, about their intended inflow. Accordingly, surveys of intentions have an opposite interpretative error, one of outflow bias.
I like this bathtub model of migration, which forces the analyst to disaggregate the flows and consider how bias might be distorting the picture. In the Rust Belt, most assume brain drain is driving the population loss (outflow bias) when the data often reveal weak inflow. Residents don't think about newcomers failing to show up in their neighborhood. They notice the people who leave. As for city revival narrative, stronger inflows receive all the ink. That too makes sense. Something not happening hides behind something that is happening. Why would anyone consider the possibility of a clogged outflow?
The specter of inflow bias and outflow bias puts coverage of population dynamics in a different light. Now, all I see are bathtubs. Demographic hysteria in the Bay Area:
More than one-third of Bay Area residents say they are ready to leave in the next few years, citing high housing costs and traffic as the region's biggest problems, according to a poll released Monday.
Hey Dowell, what does the bathtub say about San Francisco dying? "[S]urveys of intentions have an opposite interpretative error, one of outflow bias."
The headlines say nothing about a collapsing inflow. How could we conduct a survey of future intentions to move to the Bay Area? Furthermore, we don't know if those who intend to leave will actually move to another region. This story suffers from outflow bias.