Examining California Population Changes

                There has been speculation that California may lose a Congressional Seat once the 2020 Census is completed. The data provided by the US Census Bureau can be used to analyze the extent of population changes throughout the state by using data from the 2010 US Census and the bureau's published 2018 population estimates over the approximately 8-year span.1 The published estimates are limited as to the data provided and the smallest geographic area provided is at the county level. While the American Community Survey (ACS) provides more granular estimates with respect to both data and geographic boundaries. However, the ACS estimates are not pinpointed to a certain time, but fall within a range of one or five years. The 1-year ACS estimates are not provided for all counties because some do not meet the required population threshold. The 5-year ACS estimates would provide more detailed data, but the uncertainty within the time interval would result in less certain conclusions. For the sake of this analysis more recent results at the county level are more appropriate to get an idea of the population trends at a state-wide level.

The population in the state increased by 5.8% over the time span, but the population did not change uniformly. The population has declined in the northern and eastern parts of the state; Lassen County had the largest population decline at 11.7%. The population in the southern and western parts of the state, especially around the Bay Area, have grown. Four counties saw population growth over 10%: Placer (12.8%), Riverside (11.9%), San Benito (11.3%), and Alameda (10.4%).

The counties that saw the largest population declines tend to have higher median ages as well; this would suggest that the counties that have had a population decline will continue to experience a decrease in population. The map of median age by county looks almost like a mirror image of the map of population change with the counties with population growth skewing younger and vice versa. Sierra County had the oldest median age at with 55 years, while Yolo, Tulare, and Merced Counties were the youngest at 31 years. 

The population of California got older from 2010 to 2018, with the median age increasing from 35.2 years to 37.0 years. This increase is not surprising, as the median age of the United State population increased by .8 years over the same time span, from 37.2 years to 38 years. As the map below shows, there is no obvious trend in the change in median age relative to population change. That said, the northeastern part of the state had the largest increase in median age. Only two of the fifty-eight counties got younger over the time span, San Francisco by .5 years (38.5 to 38) and Butte by .2 years (37.2 to 37). Alpine County had the largest increase in median age at 4.3 years (46.7 to 51).

1 The 2010 Census is for 4/1/2010, whereas the 2018 estimates are for 7/1/2018.

The code used to pull the data and create the visualizations can be accessed at https://github.com/pfmccull/California-Population-Changes