Estimating Community Water System Populations

Population Methodology – 2016 to Present

A state’s individual control factor for adjusting the CWS population estimate is calculated by combining two established estimates: the United States Census Bureau (USCB)¹ State Population Estimates, and the US Geological Survey (USGS)² estimate for the Percent of State Population on CWS. The USCB publishes state population estimates yearly and has state-of-the-art population projections. The USGS is published every 5 years and includes national water use data, including the estimated population in each state that is served by public water systems.

The Controlled State CWS Population P_i,year for state i is equal to the USCB State Population Estimate for that year multiplied by the most recent USGS Percent of State Population on CWS estimate.

P_i,year = USCB State Population Estimate_i,year * (USGS Percentage of State Population on CWS_{i,most recent 5 year estimate})

The Control Factor c_i,year for state i is equal to the Controlled State CWS Population P_i,year divided by the Total Reported CWS Population for that state found in CDC Water Fluoridation Reporting System (WFRS).

$c_{i, y e a r} = \frac{P_{i, y e a r}}{{(T o t a l R e p o r t e d C W S P o p u l a t i o n)}_{i, y e a r}}$

All individual CWS populations in the WFRS in state i are then multiplied by the Control Factor, c_i,year . This produces a proportionately controlled CWS population for each individual CWS within that state.

The results of this calculation procedure are:

A Controlled State CWS Population (P_i,year) for each state i.
Proportionately adjusted/controlled individual CWS populations within each state.

Finally, the estimated population receiving fluoridated water within each state i is calculated by summing the controlled CWS populations for all fluoridated systems in state i.

It is important to note that CWS populations and fluoridated populations will vary based on the adjustment methodology, and current estimates should not be considered directly comparable to those of previous years using previous methods. However, the percentage of state populations on CWS receiving fluoridated water will remain comparable across old and new methodology as both the state CWS population and the state fluoridated population are adjusted by the same set proportion, regardless of methodology.

Example

In 2016, the published USCB State Population Estimate for state i was 4,258,200. Hypothetically, the USGS most recent estimate of Percent of State Population on CWS in State i was 93.1% (released in 2010). The Controlled State CWS Population P_i,2016 is calculated as follows:

P_i,2016 = 4,258,200* 0.931 = 3,964,384.2

The Total Reported CWS Population for State i found in WFRS was 5,025,123. The Control Factor for State i for that year then is calculated as:

$c_{i, 2016} = \frac{3,964,384}{5,025,123} = 0.789$

All individual CWS populations in State i are proportionately adjusted by multiplying by this Control Factor c_i,2016 = 0.789. Each controlled fluoridated CWS population is summed to determine the total state fluoridated population.

The same result is obtained for the fluoridated state population by summing the state reported fluoridated population and then multiplying by the control factor as shown below.

The state reported fluoridated CWS population for state i is 2,564,300. The controlled fluoridated, 2016 CWS population is then the following: PF_i,2016 = 2,564,300 * 0.789 = 2,023,233

The percentage of population on CWS receiving fluoridated water would be calculated as:

$% = \frac{2,023,233}{3,964,384} * 100 = 51 %$

Population Methodology – 2000 to 2014

Prior to the implementation of the 2016 Control Factor, CDC used corrective methodology to adjust the state-estimated CWS populations in states that reported having more customers than the US Census Bureau population estimates.

Using this method, the Total Reported CWS Population would be adjusted down to appropriate levels (limited to the U.S Census Bureau State Population Estimate). An adjusting factor was then used to determine the total Fluoridated CWS Population for that state.

The adjusting factor for state is equal to the USCB State Population Estimate divided by the Total Reported CWS Population for that state.

$d_{i} = \frac{{(U S C B S t a t e P o p u l a t i o n E s t i m a t e)}_{i}}{{(T o t a l R e p o r t e d C W S P o p u l a t i o n)}_{i}}$

The total state population on CWS receiving fluoridated water is then calculated as:

PF_i = d_i * (Total Reported F Population)

The final result of this population methodology were:

A Controlled State CWS Population for each stated state i that had a total reported CWS population greater than the USCB estimate (which was equal to the USBC estimate).
Proportionately adjusted state population on CWS receiving fluoridated water.

The most recent published data are available at:

Census Population Count 2018. Annual Estimates of the Population for the United States, Regions, States, and Puerto Rico: April 1, 2010 to July 1, 2018. Source: US Census Bureau, Population Division. Release Date: December 2018. Available at https://factfinder.census.gov/faces/tableservices/jsf/pages/productview.xhtml?pid=PEP_2018_PEPANNRES&src=pt.
US Geological Survey: Maupin, M.A., Kenny, J.F., Hutson, S.S., Lovelace, J.K., Barber, N.L., and Linsey, K.S., 2014, Estimated use of water in the United States in 2010: US Geological Survey Circular 1405, 56 p., https://dx.doi.org/10.3133/cir1405.