Measures of Disease Frequency & Morbidity

Define the measured event, population size, and observation period to assess disease frequency, and express rates with multipliers per 100,000 or per 1,000 for clarity.

Measure incidence as the frequency of transitions from well to ill, using incident cases or incidence rate with person-time data, and express it as cumulative incidence.

Incidence rate measures disease risk by counting new cases during a time period and relating them to the population at risk or to person-time in open or dynamic populations.

Calculate the incidence rate by dividing new cases by the at-risk population over a period, expressing per thousand, with the denominator including only those at risk.

Compute incidence rate using person months at risk across study periods, accounting for lost to follow up and health events like cancer to express cases per 100 person months.

Define cumulative incidence as the incidence proportion or risk that healthy people will develop disease over a specified time, excluding those with prior disease and using a defined at-risk population.

Explore the difference between cumulative incidence and incidence rate, comparing risk over a time interval with incidence density, and learn when to use each measure in population health.

Explore attack rates in outbreak settings, including overall attack rate, food-specific attack rate, and secondary attack rate defined by exposure during the incubation period.

Demonstrate calculating attack rates from outbreak data, including the diary attack rate for those who ate both ice cream and pizza, and the secondary attack rate in a measles household.

Prevalence defines how common a disease is in a defined population at a given time, expressed as a percentage. It includes new and existing cases, with point and period prevalence.

Compute the prevalence rate as the number of existing arthritis cases (numerator) per population (denominator) on a specific date, with a multiplier to suit stakeholders.

Explain how prevalence guides public health work when incidence data are unavailable, using asthma and obesity trends measured by self-report across states and time.

Demonstrates calculating surveillance, point prevalence, and cumulative incidence, using at-risk denominators; finds 27% existing cases, 17% point prevalence, and 12% cumulative incidence.

Calculate the point prevalence for surveillance on April 1, 2005 by dividing seven ill people by the population of 18 and multiplying by 100 to obtain 38.89 percent.

In this example, compute period prevalence during the surveillance period by dividing the number ill by the total population, yielding 50 percent.

Explore how incidence drives prevalence in dynamic populations, where new cases, immigration and emigration shape the number of prevalent cases. Mortality and recovery shorten duration and can decrease prevalence.

Explain morbidity measures by detailing incidence and prevalence concepts, including point prevalence and surveillance calculations over a calendar year using a single population with eight cases.

Read between the lines and examine external factors to interpret disease frequency and morbidity. Recognize that prevalence equals incidence times duration; higher prevalence may reflect longer survival and socioeconomic factors.

Establish a clear case definition to decide who counts in the numerator, specifying inclusion and exclusion criteria to differentiate similar symptoms, prevent data misinterpretation, and ensure consistent surveillance.

Explore how epidemiology measures disease frequency to inform health service planning, highlighting incidence, prevalence, and morbidity while noting data quality issues and error sources in developing countries.