Study population and setting
This study used aggregated, anonymized smartphone data from January 6 to May 3, 2020 to test the hypothesis that differences in physical distancing previously observed between higher and lower income neighborhoods in the US were explained by work-related demands outside the household. The authors analyzed daily mobility data from approximately 19 million smartphones in all 50 states. The primary outcome was the proportion of smartphone users who spent all day at home, where home was defined as the place where a user spent the most nights over the last 6 weeks. A secondary outcome was the proportion of users who went to work on a given day. Leaving the home for work was defined as stopping at a single location for three or more hours between the hours of 8 am to 6 pm or going to four or more locations for less than 20 minutes each (e.g., delivery work). Travels to points of interest outside of work were also analyzed as a secondary outcome, including visits to parks, playgrounds, carryout restaurants, places of worship, convenience stores, and liquor stores. The primary exposure was median household income in census block groups (CBGs) categorized into quintiles. The impacts of state-level physical distancing policies (stay at home orders and non-essential business closures) on the primary and secondary outcomes by income quintile were analyzed using difference-in-difference linear regression models with fixed effects for state and calendar date.
Summary of Main Findings
There were a total of 210,119 census block groups and points of interest included in the analysis. Increases in physical distancing were observed over the analysis period for all income groups, although increases were more pronounced in higher income neighborhoods. The proportion of days spent at home increased by 11 percentage points in the lowest income group, while it increased by 27 percentage points in the highest income group (p<0.0001; 95%CI: 16.0-16.1). Changes in work-related mobility appeared to explain much of this difference: the highest income group went from working the most days outside the home to the fewest after physical distancing policies were implemented. Visits to all POIs declined in all income groups. Visits to places of worship were the only point of interest with a clear income gradient associated with a decline in visits, with fewer visits in higher income groups.
This study used a large, well-characterized mobile phone dataset from diverse geographic locations in the United States. Outcomes and exposures were clearly defined, and analyses were conducted using standard, easily interpretable analytic techniques.
The number of smartphones per census block group was strongly associated with the median household income level, with fewer smartphone users in lower income census blocks. This may have biased results, particularly if those with smartphones in lower income census blocks were substantially more likely to work outside the home than those without smart phones. Younger smartphone users may have had changes in education-related mobility that were counted as work-related; if those from wealthier areas stayed home from school more frequently, the relationship between income and work-related mobility would be overestimated. Working outside the home was crudely defined as time spent outside the household during daytime hours, and did not include individuals who worked outside the home after 6 pm and before 8 am. Smartphone locations were measured at irregular intervals; aggregated mobility data could systematically misrepresent work-related or home-related behavior.
This study is the first to assess whether disparities in physical distancing observed between higher and lower income neighborhoods is explained by work outside the home.
This review was posted on: 19 December 2020