Problem setEpidemiological measures

Epidemiology (PUBH – 6035 – 2) Module 2

Two datasets with dates that matter. Prevalent cases stay out of the incidence denominator, and person-time has to be accumulated subgroup by subgroup rather than assumed.

Editorial process

Last reviewed · August 22, 2026

Written by

Maren Caldwell

MSN, RN, CNE

Medical-surgical nursing, pharmacology and NCLEX preparation

Reviewed by

Dr. Tessa Redmond

DNP, RN, CNE

Evidence-based practice and clinical education

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Interpret the task
Assignment instructions
Deliverables
Proposed outline
Research plan
Reference shortlist
Final review

Prevalent cases are not at risk, and dates are data

Two rules decide almost every mark here, and both are about denominators. The first is that prevalence and incidence use different ones. Prevalence at a point in time is all existing cases divided by the population present at that moment, so the five children diagnosed before January counts toward prevalence throughout. Incidence measures new cases among those who could still become cases, so those same five are excluded from the population at risk, which starts at ninety-five and not one hundred. Mixing the two is the commonest error and it changes every subsequent number. The dates are data rather than decoration: a case arising on March 1st and one arising on July 1st contribute different amounts of at-risk time, and a denominator that ignores when things happened will be wrong even when the case count is right. Read each sentence of the vignette as either a numerator event or a denominator adjustment.

The second is that person-time has to be accumulated group by group rather than assumed. Each subgroup contributes only the time it was actually observed and still at risk: those who developed asthma contribute time up to their diagnosis, those lost to follow-up contribute time up to that date, and the rest contribute the full period. Add the groups, express the answer in the units the question asks for, and put the incidence rate over person-time rather than over a head count. Two conventions are worth stating rather than assuming, because markers differ: whether a case is counted as at risk up to the date of diagnosis or to the midpoint of the interval, and how those lost to follow-up are handled. Show your working with the units at every step, and write the interpretation sentence for each result, since a rate without units and a population attached is not an answer.

Likely learning objectives

Use the correct denominator for prevalence and for incidence.
Exclude prevalent cases from the population at risk.
Accumulate person-time by subgroup with actual observation periods.
State the conventions used rather than leaving them implicit.

Assignment instructions

Read the full question

Review every instruction before using the planning guidance that follows.

Question 1 Suppose that you are following a group of children for the development of asthma over a one- year period. You identify 100 children on January 1st, screen them for asthma, and set up a monitoring program to check on their status on a monthly basis. Five children are considered prevalent cases because they were diagnosed with asthma before January 1st. Ten children develop asthma on March 1st and another ten children develop asthma on July 1st. Another 10 children who remain healthy were followed for six months and then were lost to follow-up. All of the remaining children did not develop asthma and were not lost to follow-up. Follow-up ended on December 31st. · What was the prevalence of asthma on June 1st? · What was the prevalence of asthma on September 1st? · How many person-months of observation were accrued by this population? · What was the incidence rate of asthma in this population for the one-year period Question 2 A population of 1,000 people is monitored for a year for the development of measles. No one has measles at the start of the investigation. Thirty people develop measles on June 30 and twenty people develop measles on September 30. Eight people are lost to follow-up on March 31 and twenty-four people are lost to follow-up on November 30. None of those lost to follow-up had developed measles prior to becoming lost. Assume that you can only get measles once. · What is the cumulative incidence of measles in this population? · What is the incidence rate of measles? · What is the prevalence of measles on July 1? Epidemiology (PUBH – 6035 – 2) Module 2 Question 3 · Which type of measure of disease frequency best describes each of the following scenarios? · Percentage of students enrolled in a college who developed influenza during the spring semester of 2012. · Percentage of students enrolled in an epidemiology class who had sore throats on the first day of class. · Percent of breast cancer patients who underwent mastectomy during 2012. · Percent of men found to have high blood pressure at their yearly physical. · Number of newly-diagnosed cases of AIDS in a year per 100,000 persons. · Percent of infants born with spina bifida out of 1,000 live-born infants. · Percent of drivers found to be legally drunk at the time of their car accident. Epidemiology (PUBH – 6035 – 2) Module 2

Turn the brief into deliverables

01Point prevalence at each date requested, with the denominator shown.
02Total person-months of observation, accumulated by subgroup.
03An incidence rate expressed per person-time with units.
04The same calculations for the second dataset.
05A stated convention for handling diagnosis dates and losses.
06An interpretation sentence for each result.

Set up the denominators, then compute each measure

Set up the population

Separate prevalent cases from the population at risk before calculating anything.

What the assessor is likely looking for

A population at risk that excludes existing cases.

Prevalence at each date

Compute point prevalence with the numerator and denominator shown.

What the assessor is likely looking for

A denominator adjusted for who is present at that date.

Accumulating person-time

Add the time contributed by each subgroup up to event or loss.

What the assessor is likely looking for

Losses to follow-up contributing partial time.

The incidence rate

Divide new cases by person-time and express with units.

What the assessor is likely looking for

A rate per person-time rather than per person.

Stating your conventions

Say how diagnosis dates and losses were treated and why.

What the assessor is likely looking for

A convention stated rather than silently applied.

Where the measure definitions are documented

Recommended databases

NCBI Bookshelf
PubMed Central
CDC
University Library

Search sequence

1.Read the definitions of prevalence and incidence before starting the arithmetic.
2.Confirm how person-time is accumulated for events and losses.
3.Check the convention your course uses for mid-interval contributions.
4.Look at a worked example expressed in the same units as the question.

Reference shortlist

These are authoritative starting points, not a ready-made bibliography. A qualified reviewer must confirm that each source fits the assignment and supports the claim beside which it is cited.