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.

Measures of morbidity quantify disease frequency in a population by distinguishing incidence, the rate of new cases, from prevalence, the proportion of existing cases and disease duration.

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.

Calculate the incidence rate in a dynamic population by summing each person’s time at risk to form total person-time, using incidence density with new cases as the numerator.

Calculate person-time at risk by summing each participant's observation time to yield the total person years used as the study's denominator.

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.

Compute cumulative incidence of schistosomiasis in 2015 by 22 cases divided by a 700,000 at-risk population, multiplied by 100,000, yielding 3.14 per 100,000, assuming everyone is at risk.

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.

Differentiate point prevalence from period prevalence: point prevalence is how common a disease is at a specific time, while period prevalence covers a time interval; surveillance combines prevalence with incidence.

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.

Compute the period prevalence for the year, determine the point prevalence at the start, and estimate the one-year incidence using 150 people with 25 existing cases and 15 new cases.

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

Calculate incidence rate per hundred from oct 1, 2004 to sep 30, 2005 using midpoint population; assess point and period surveillance for a 20-person, 12-month study with ten new cases.

Calculate the incidence rate from October 1, 2004 to September 30, 2005 using the midpoint population; four new cases over 18 people times 100 yields about 22 per 100.

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.

Compare incidence and prevalence with a month-by-month scenario, showing incidence counts new cases while prevalence tracks existing cases, reflecting acute versus chronic disease focus in public health planning.

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.

Visualize incidence and prevalence with a CDC AIDS graph, showing yearly incident cases and the alive versus dead status to illustrate cumulative incidence.

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.

Identify diverse data sources used by epidemiologists, including disease reporting, insurance records, hospital and clinic reports, and industry or school records, while validating data and avoiding nonrepresentative samples.

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 diagnostic criteria for rheumatoid arthritis affect numerators and prevalence, and why precise case definitions shape morbidity data in population health.

Identify who belongs in the numerator and how interview data shape incidence and prevalence estimates. Explore how questions and interviewers introduce error.

Denominators must include individuals at risk, as hysterectomy removes uterine cancer risk, making corrected rates higher. Epidemiologists confront data limitations and incomplete records while ensuring accuracy to reflect the population.

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.

Explore measures of disease frequency and morbidity, and thank you for listening as Professor Candelario wishes you well.