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FAQs (Data Related) Data Analysis

  1. Is it acceptable to pool years of data with identical BRFSS arthritis questions to obtain a sufficient sample size for population subgroups? Are we really double counting when we do this?
  2. Is it possible to look at trends in prevalence since the BRFSS arthritis questions changed in 2002?
  3. How is prevalence defined?
  4. How is incidence defined?
  5. What is the incidence of arthritis?

  1. Is it acceptable to pool years of data with identical BRFSS arthritis questions to obtain a sufficient sample size for population subgroups? Are we really double counting when we do this?

    We recommend you either pool data from 1996 to 2001 when one set of questions was used, or from 2002 forward when a different set of questions was used. Pooling data from these two separate periods is not legitimate because the underlying survey questions were different (see #2 below). Combining two or more years of data within a period can generate more precise prevalence estimates. If you divide the final weight by the number of years you pooled, there will not be double or triple counting.

    Stargin in 2011 the BRFSS changed the order of arthritis questions by placing the case definition question in a Core Chronic Health Conditions Module and leaving the remaining 4 questions in the separate Core Arthritis Burden Module. The effect of changing the order of the questions on participant responses is not clear but should be considered when combining 2011 and later data with dat from earlier years.

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  2. Is it possible to look at trends in prevalence since the BRFSS arthritis questions changed in 2002?

    Estimates of trends can begin in 2002 or can run from 1996–2001 but not both because in 2002 both the questions used and the case definition changed. This makes it impossible to know if any change in prevalence from 2001 to 2002 is real or due to different methods. Regarding trends, it is reasonable to expect an increase in arthritis prevalence over time because the population is aging and we know there are higher rates of arthritis among older persons. Further information is available on: arthritis case definition changes, an overview of BRFSS arthritis question changes and BRFSS questions listed by year.

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  3. How is prevalence defined?

    Prevalence is the number of all persons with arthritis in a given population at a given time. Prevalence is sometimes used to mean prevalence “rate” (really a proportion). Prevalence rate has a numerator of the total number of all persons with arthritis at a given point in time (e.g., 2004) divided by the total number of persons in that defined population (e.g., persons age 18 years and older). This is the most common estimate of arthritis burden.

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  4. How is incidence defined?

    Incidence is the number of new persons whose arthritis begins during a given time period in a given population. This is a rarely used estimate of arthritis occurrence because the date of onset of new cases is difficult to determine.

    Note: Incidence cannot be calculated from NHIS or BRFSS data.

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  5. What is the incidence of arthritis?

    Estimating the incidence of arthritis, that is, the number of new cases of arthritis in a defined population over a defined time (usually a year), is very difficult. It requires knowing the disease status of everyone in the defined population at the start of the defined time period (to remove those with existing cases of arthritis from the estimates) and then counting every new case that occurs till the end of the time period.

    These challenges have meant that incidence studies have been done in fairly small populations in geographically small areas over relatively short time periods and, thus, are not generalizable to the U.S. population. Consequently, we have no national estimate of arthritis incidence.

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