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.
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.
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.
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.
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.