NUR 520 Module 3 Measures of Disease Frequency Example

Same Counties, Different Rankings: Crude and Age-Adjusted COPD Prevalence in New Hampshire

[Student Name]

Southern New Hampshire University

NUR 520: Epidemiological and Biostatistical Applications in Healthcare

Module Three Assignment

[Instructor Name]

[Date]

Same Counties, Different Rankings: Crude and Age-Adjusted COPD Prevalence in New Hampshire

Measures of disease frequency answer the question of how common a condition is, but the answer depends on the measure chosen. This paper compares crude and age-adjusted prevalence of diagnosed COPD among adults in New Hampshire's ten counties, using CDC's PLACES 2025 county estimates (Centers for Disease Control and Prevention [CDC], 2025). It shows that Coös County ranks highest under both measures, which strengthens the case that its burden is not simply a result of an older population, while other counties move several places, which illustrates why comparisons between places must account for age.

Prevalence, Not Incidence

Two families of measures describe how common a disease is. Incidence tracks the rate at which people who were free of the disease develop it during a stated interval, while prevalence captures everyone who has it, whether diagnosed last week or twenty years ago, as a share of the population at one time (Celentano & Szklo, 2019). For a chronic, largely irreversible disease such as COPD, a prevalence figure blends two things at once, the pace at which people are newly diagnosed and the years they survive afterward. It is the right measure for planning ongoing services such as pulmonary rehabilitation and inhaler education, because it describes how many people need them now. Incidence would be better for evaluating whether prevention is working, but county-level COPD incidence is not publicly available, which is itself a limit worth stating.

Crude and Age-Adjusted Prevalence

Crude prevalence is the proportion of all adults in a county who have diagnosed COPD. It describes the real burden in that population. Because COPD becomes more common with age, an older county can post a higher crude figure even when, age for age, its residents carry no extra risk at all. Age-adjusted prevalence removes that effect. In the direct method, the age-specific rates observed in each county are applied to a single standard population, so every county is compared as if it had the same age structure. Federal statistics commonly use the projected 2000 U.S. population as the standard, which allows comparisons across places and years (Klein & Schoenborn, 2001). PLACES reports both measures for every county.

The County Comparison

Table 1 lists the ten counties with both measures and their rank under each.

Table 1

Estimated Prevalence of Diagnosed COPD Among Adults by New Hampshire County, Crude and Age-Adjusted

Note. Data from CDC PLACES, 2025 release, model-based estimates. Ranks were assigned for this paper; ties share a rank.

Why the Rankings Change

Two counties illustrate the effect of age. Carroll County, a lakes and mountains region popular with retirees, ties for third on crude prevalence at 7.5% but falls to eighth after age adjustment, at 5.2%. Much of its crude burden reflects an older population rather than higher age-specific risk. Strafford County, home to a large state university and a younger population, moves the other way, from eighth on crude prevalence to fifth after adjustment, suggesting that its age-specific risk is higher than its crude figure implies. Coös County remains first under both measures, although its figure falls from 9.5% to 7.1% after adjustment. The county's older age structure explains part of its burden, but not all of it: even at a standard age structure, Coös residents are estimated to have the highest prevalence in the state.

Which Measure to Use for What

Each measure answers a different planning question. Crude prevalence tells a nurse leader in Coös how many adults actually live with diagnosed COPD, which is what matters for sizing a pulmonary rehabilitation program or estimating how many patients a care manager might follow. Applied to the county's roughly 26,000 adults, a crude prevalence of 9.5% suggests about 2,500 adults with diagnosed COPD, a calculation made for this paper from the published estimates. Age-adjusted prevalence tells the same leader whether Coös residents face higher risk than people elsewhere at the same ages, which matters for arguing that the county needs more prevention resources than its size alone would justify. Using the wrong measure for the question can mislead: crude figures exaggerate risk in older counties, while adjusted figures understate how many people need care there.

Age-Specific Rates as a Next Step

Age adjustment summarizes a comparison in a single number, but it can hide important detail. Two counties with the same age-adjusted prevalence could differ sharply in one age group, for example if one has unusually high COPD among adults in their fifties because of past occupational exposure. PLACES does not publish county estimates by age group, so that detail is not available from this source. A nurse leader could obtain it from local clinical data, counting patients with a COPD diagnosis by age group in the county's health center and hospital records, while recognizing that those counts describe only people who reached care. If the county's excess were concentrated in adults under 65, the response would lean toward workplace and cessation programs; if it were concentrated among older adults, toward rehabilitation, home oxygen support and transitions of care.

Limits of the Comparison

These are model-based estimates, each with a confidence interval, so small differences between counties may not be meaningful; the next module examines that question directly. The estimates describe diagnosed COPD only. And age adjustment removes differences due to age, but not those due to smoking, occupation or access to diagnosis, which are the very factors a nurse leader would want to investigate next.

Conclusion

Crude and age-adjusted prevalence rank New Hampshire's counties differently because the counties differ in age. Coös County ranks highest either way, which strengthens the case that its COPD burden reflects more than an older population. For planning services, the crude figure shows how many people need care; for arguing about risk and prevention resources, the adjusted figure provides a fair comparison.

References

Celentano, D. D., & Szklo, M. (2019). Gordis epidemiology (6th ed.). Elsevier.

Centers for Disease Control and Prevention. (2025). PLACES: County data (GIS friendly format), 2025 release [Data set]. https://data.cdc.gov/d/i46a-9kgh

Klein, R. J., & Schoenborn, C. A. (2001). Age adjustment using the 2000 projected U.S. population (Healthy People 2010 Statistical Notes No. 20). National Center for Health Statistics. https://www.cdc.gov/nchs/data/statnt/statnt20.pdf