Factors that affect prevalence

Calculation of Person-Time

10 people followed for 3 years= 30 person-years or 360 person-months.

Have you ever had asthma?

Cumulative or Lifetime incidence

Challenges with the Numerator: Who has disease?

1) Diagnostic criteria can strongly impact rates 2) Can be differences in case definitions 3) Case definitions may change over time. 4) May be changes in quality of screening services.

Prevalent proportions are decreased by

1) Immigration of healthy persons 2) Emigration of persons with disease 3) Improved cure rates 4) increase death rates 5) Decreases in incidence (occurrence of new cases) 6) Shorter duration of the disease

Challenges with the Numerator: Who should be counted?

1) Is case finding accurate and inclusive? How large is effort to find all cases? 2) Case finding by interview may yield inaccurate results.

Challenges with the Denominator

1) Selective undercounting- Who doesn't get counted 2) Census geographic boundary changes 3) Changes in defined populations over time

Prevalent proportions are increased by

1) immigration of cases or those with a high potential to become cases (elderly) 2) emigration of healthy individuals 3) increases in the duration of the disease (earlier detection, better treatment, reduced fatality, etc.) 4. Increases in incidence (occurrence of new cases)

Did you have your first episode of asthma this year?

Incidence

Incidence Density

Incidence density = # of new cases of disease in a time period/ person-time for the risk for the same time period.

Have you had asthma during the last year?

Period Prevalence

Do you currently have asthma?

Point Prevalence

The relationship between Incidence and prevalence

Prevalence = Incidence x Duration