Prevalence

Calculation of prevalence rates and number of cases

To estimate out the number of people in each county with the chronic condition, we multiplied the population by the prevalence rate:

Number of people in the county for each age, gender, and ethnicity
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Prevalence rate for each condition by each age, gender, and ethnicity
= Number of people with the chronic disease in the county by age, gender, and ethnicity

Estimates of the prevalence of the six chronic diseases of interest for California were derived from a variety of data sources, including AskCHIS online query system for the California Health Interview Survey, SEER-Medicare data, and the Centers for Disease Control and Prevention. State-level prevalence rates for (a) arthritis, (b) asthma, (c) cardiovascular disease, and (d) diabetes were obtained from the 2019-2020 California Health Interview Survey (CHIS). The CHIS is a representative population-based, random-dial, health survey of non-institutionalized individuals in California which is used to estimate prevalence rates for various health diseases at the state-level, and for large and medium size counties at the county level (smaller counties are grouped together). State, regional, and county-level estimations of various diseases and health related behaviors surveyed in the CHIS can be obtained from the online webtool AskCHIS. Because the 2019-2020 CHIS sample size was too small to obtain county-level prevalence rates for each age strata by race-ethnicity and gender, the analysis was conducted with region prevalence rates (rather than county-level rates).
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Cancer prevalence rates were obtained from the National Cancer Institute Surveillance Epidemiology and End Result (SEER) data. Cancer prevalence for each age strata by race-ethnicity and gender were calculated for each of the 58 California counties using the program Seer Stat and 2020 SEER. The county-level prevalence rates were estimated for the following strata in order to match the age groups used by SEER: 0-19, 20-44, 45-64, 65-79, and 80 or older. Due to small sample sizes, no estimates were available for small counties, or small demographic subgroups within counties. Small counties were therefore combined and average rates applied to each small county. Two adjustments were made to the CHIS. First, CHIS provides data by age brackets:

• 18 – 24
• 25 – 39
• 40 – 64
• 65 – 79
• 80+

Because the Costs Calculator and Return on Investment Tool requires data for each individual age, the rates for each age were extrapolated from the available data. The process of extrapolation involved:

• Finding the midpoint of each age group.

• For instance, 21 years old is the midpoint for the 18 to 24 age group and 32 years old is the midpoint for the 25 to 39 age group.

• Assuming that the rate for the age group was the rate for the midpoint.

• For instance, the rate for a 21 year old (yo) was the rate for the age group 18 to 24. The rates for a 32 yo was the rate for the 25 to 39 age group.

• Extrapolating the rate of each additional age by assuming a linear relationship between the rates of the age groups.

• For instance, the difference between the rate for a 32 yo and a 21 yo was calculated, and the rate for a 22 yo was:

• Rate for a 22 yo = Rate for a 21 yo + (Rate 32 yo - Rate 21 yo) /11 × (22-21)

• This was done for each of the ages. Note that it was assumed that the starting point for an 18 yo was assumed to be 0% and then adjusted accordingly.

The second way that estimates were adapted is to account for the difference in ethnicities reported in CHIS from the Census ethnicities and the ethnicities included in the CDC Cost Calculator. A comparison of the ethnicities is in the table below. The CHIS category of  “American Indian/Alaska Native and Other” was assumed to be broadly consistent with the category “Other Ethnicities” in the CDC Calculator.

Table xxx. Ethnicities in data sets