MPH 530 Week 1 Measures of Disease Frequency Example

Counting Correctly: Prevalence, Incidence and Mortality Rates Worked Through for Diabetes and Overdose in a Southern Colorado County

[Student Name]

University of Phoenix

MPH/530: Epidemiology Concepts and Public Health Diseases

Week 1 Assignment

[Instructor Name]

[Date]

The county health department, its cohort and local counts are composites written for a model paper; state and county estimates come from the CDC data sets cited, retrieved in September 2026.

At a budget hearing, a county commissioner in a composite southern Colorado county asked the health department's epidemiologist a reasonable question: why does the department report diabetes as a percentage but overdoses as deaths per 100,000, and which number tells me the bigger problem? The epidemiologist prepared this paper to explain the measures of disease frequency and work through them with local and state data.

Three Families of Measures

Measures of disease frequency fall into three families. Prevalence answers what share of a population currently has a condition. Incidence answers how fast people are newly developing it. Mortality measures answer how many are dying. Each answers a different question.

Prevalence

Prevalence is the number of existing cases divided by the population at the same time. It tells planners how much burden exists now, for example how many people need diabetes care, medicines and supplies.

Incidence

Incidence counts new cases. The incidence proportion, also called cumulative incidence or risk, puts the people who became cases over a stated period over everyone who could have become a case when the period began. The incidence rate instead puts new cases over the summed time each person was actually watched, which copes with people joining or dropping out at different points.

How Prevalence and Incidence Relate

A methods review explains that prevalence reflects both incidence and duration: in a steady state, prevalence is approximately incidence multiplied by average disease duration, so conditions with long survival can have high prevalence even when new cases are not especially common (Noordzij et al., 2010). Diabetes, which people live with for decades, has high prevalence for this reason.

Diabetes Prevalence in the County

CDC's 2025 county estimates for Pueblo County, on which the composite county is modeled, put age-adjusted diagnosed diabetes prevalence among adults at 10.4% (Centers for Disease Control and Prevention, 2025). With roughly 131,000 adults in a population of about 169,000, that implies roughly 13,600 adults with diagnosed diabetes if applied to the crude count, an approximation that is useful for planning services.

Why Age Adjustment

Age-adjusted prevalence recalculates what prevalence would be if the county had a standard age distribution. Because diabetes rises with age, a county with more older adults would show higher crude prevalence. Age adjustment allows fair comparisons with other counties and with the state; crude counts are better for planning local services.

Incidence From the Prevention Cohort

The department followed 402 adults with prediabetes enrolled in its prevention program for one year. During the year, 14 were newly diagnosed with type 2 diabetes. The incidence proportion was 14 divided by 402, or 3.5% over one year.

Incidence Rate With Person-Time

Not everyone was followed for the full year: some moved or left the program. Together the 402 participants contributed 351 person-years of follow-up. The incidence rate was 14 new cases divided by 351 person-years, or 4.0 per 100 person-years. The rate is slightly higher than the proportion because it accounts for time actually observed.

Overdose Mortality

CDC's provisional counts show 1,819 drug overdose deaths in Colorado in the 12 months ending December 2025 (Centers for Disease Control and Prevention, 2026). With a state population of roughly 6.0 million, the crude overdose mortality rate was about 30 deaths per 100,000 residents per year. Rates per 100,000 are used for deaths because deaths are rare relative to the population; a percentage would be a small, hard-to-read number.

Proportionate Mortality

The same data show 45,870 deaths from all causes in Colorado over that period, so overdoses accounted for 1,819 divided by 45,870, or 4.0% of all deaths. Proportionate mortality tells the share of deaths from a cause, not the risk of dying from it. A cause can make up a larger share of deaths either because it rose or because other causes fell.

Drug-Specific Counts

The provisional data also specify drugs involved. Synthetic opioids other than methadone were involved in 1,044 deaths and psychostimulants such as methamphetamine in 879, about 57% and 48% of overdose deaths. The percentages add to more than 100% because many deaths involve more than one drug.

Years of Life Lost

Another way to compare the two problems is premature mortality. Overdose deaths in Colorado occur mostly among adults aged 25 to 54, while diabetes deaths occur mostly after 65. Measures such as years of potential life lost weight deaths by how early they occur, which is why overdoses rank higher on premature mortality than their share of total deaths suggests.

Rates Over Time

A single year's rate is a snapshot. The epidemiologist showed the commissioner how Colorado's overdose deaths moved over five years, from 1,917 in 2021 to 1,640 in 2024 and 1,819 in 2025, and explained that comparing years requires the same measure and the same data source each time.

Which Number Shows the Bigger Problem

The commissioner's question has no single answer, because the measures answer different questions and serve different decisions. Diabetes affects far more people at any time, about one adult in ten, and drives ongoing care costs. Overdoses kill far more younger people suddenly, each death representing many years of life lost. Both measures are needed.

A Summary Table

The epidemiologist's table listed each measure, its formula, the value and its meaning: diabetes prevalence, 10.4% age-adjusted, how common diagnosed diabetes is among adults; incidence proportion, 3.5% over one year, the risk of diabetes among program participants; incidence rate, 4.0 per 100 person-years, new cases per time at risk; overdose mortality, about 30 per 100,000 per year; proportionate mortality, 4.0% of deaths.

Common Errors

Common errors include confusing prevalence with incidence, omitting the time period from rates, comparing crude rates between populations with different ages, ignoring person-time when follow-up varies and adding drug-specific percentages as if they were exclusive.

Checking the Arithmetic

Before presenting, a second epidemiologist recalculated every figure from the source files. Independent checking caught one error in an early draft, where the person-time had been entered as 315 instead of 351 years, which would have overstated the incidence rate. Simple checks prevent embarrassing corrections later.

Limits of the Data

County prevalence estimates are modeled from survey data and carry uncertainty. Provisional death counts may change as investigations are completed. The cohort is small, so its incidence estimate is imprecise.

A Plain-Language Guide for Commissioners

Prevalence tells you the size of the group that already has a condition and needs services today. Incidence tells you how fast new cases are appearing and whether prevention is working. Mortality rates tell you how many people are dying, adjusted for population size.

Conclusion

The commissioner's question had a precise answer: different measures answer different questions. Worked with local and state data, prevalence showed the burden of diabetes, incidence showed the pace of new cases in the prevention cohort and mortality measures showed the toll of overdoses. Showing each calculation, choosing the right measure and interpreting it plainly are the first skills of epidemiology.

References

Centers for Disease Control and Prevention. (2025). PLACES: Local data for better health, county data, 2025 release [Data set]. data.cdc.gov. https://data.cdc.gov/d/swc5-untb

Centers for Disease Control and Prevention. (2026). VSRR provisional drug overdose death counts [Data set]. data.cdc.gov. https://data.cdc.gov/d/xkb8-kh2a

Noordzij, M., Dekker, F. W., Zoccali, C., & Jager, K. J. (2010). Measures of disease frequency: Prevalence and incidence. Nephron Clinical Practice, 115(1), c17-c20. https://doi.org/10.1159/000286345