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## Prevalence

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**Prevalence**The presence (proportion) of disease or condition in a population (generally irrespective of the duration of the disease) Prevalence: Quantifies the “burden” of disease. - Point Prevalence - Period Prevalence**“Point” Prevalence**Number of existing cases P = -------------------------------- Total population At a set point in time (i.e. September 30, 1999)**“Point” Prevalence**Example: On June 30, 1999, neighborhood A has: • population of 1,600 • 29 current cases of hepatitis B So, P = 29 / 1600 = 0.018 or 1.8%**“Period” Prevalence**Number of existing cases Pp = -------------------------------- Total population During a time period (i.e. May 1 - July 31, 1999) Includes existing cases on May 1, and those newly diagnosed until July 31.**“Period” Prevalence**Example: Between June 30 and August 30, 1999, neighborhood A has: • average population of 1,600 • 29 existing cases of hepatitis B on June 30 • 6 incident (new) cases of hepatitis B between July 1 and August 30 So, Pp = (29 + 6) / 1600 = 0.022 or 2.2%**Prevalence**AXIOM: In general, a person’s probability of being captured as a prevalent case is proportional to the duration of his or her disease. Thus, a set of prevalent cases tends to be skewed toward cases with more chronic forms of the disease.**Mathematical Notation:A few measures used in Epidemiology**render decimal numbers • Prevalence: 58 cases of diabetes 25,000 Subjects Prevalence = 0.00232 • Incidence: 34 new cases of breast cancer 54,037 Person-Years Incidence = 0.00063**So that those numbers have a meaning, we add a population**unit: Usually a power of 10 (10N) 101 = 10 102 = 100 103 = 1000 104 = 10000 Prevalence = 0.00232 * 105 = 232 cases of diabetes/100,000 Population Incidence = 0.00063 * 105 = 63 New cases of breast cancer/100,000 Person Years**Special Types of Incidence and**Prevalence Measures INCIDENCE Mortality Rate # of Deaths over specified time Total Population Case Fatality Rate # of Deaths from a Disease # of Cases from that Disease Attack Rate # of Cases of a Disease Total Population at Risk for a Limited Period of Time PREVALENCE Birth Defect Rate # of Babies w/Given Abnormality # of Live Births**Discussion Question 4**How are incidence and prevalence of disease related?**Discussion Question 4**Prevalence depends on: - Incidence rate - Disease duration**Relationship between prevalence and incidence**WHEN (the steady state is in effect): a) Incidence rate (I) has been constant over time b) The duration of disease (D) has been constant over time: ID = P / (1 – P) P = ID / (1 + ID) c) If the prevalence of disease is low (i.e. < 0.10): P = ID .**Uses of Incidence & Prevalence Measures**Prevalence: Snap shot of disease or health event • Help health care providers plan to deliver services • Indicate groups of people who should be targeted for control measures • May signal etiologic relationships, but also reflects determinants of survival**Uses of Incidence & Prevalence Measures**Incidence: Measure of choice to: --- Estimate risk of disease development --- Study etiological factors --- Evaluate primary prevention programs**Discussion Question 5**Why is incidence preferred over prevalence when studying the etiology of disease?**Discussion Question 5**Because, in the formula: P = I x D D is related to : - The subject’s constitution - Access to care - Availability of treatment - Social support - The severity of disease**Discussion Question 5**• So prevalent cases reflect factors related to the incidence of disease (Etiological factors), AND factors related to the duration of disease (Prognostic factors) • Thus, they are not adequate for studies trying to elucidate Disease Etiology**PROBLEMS WITH INCIDENCE AND PREVALENCE MEASURES**Problems with Numerators: • Frequently, the diagnosis of cases is not straightforward • Where to find the cases is not always straightforward**PROBLEMS WITH INCIDENCE AND PREVALENCE MEASURES**Problems with Denominators: • Classification of population subgroups may be ambiguous (i.e race/ethnicity) • It is often difficult to identify and remove from the denominator persons not “at risk” of developing the disease.**Summary of Incidence and Prevalence**PREVALENCE: Estimates the risk (probability) that an individual will BE ill at a point in time • very useful to plan for health-related services and programs**INCIDENCE:**- Estimates the risk (probability) of developingillness - Measures the change from “healthy” status to illness. • Useful to evaluate prevention programs • Useful to forecast need for services & programs • Useful for studying causal factors.**SUMMARY**Cumulative incidence (CI): estimates the risk that an individual will develop disease over a given time interval Incidence rate (IR): estimates the instantaneous rate of development of disease in a population**OTHER MORTALITY MEASURES**Proportionate Mortality: Proportion of all deaths attributed to a specific cause of death: No. of deaths from disease X in 1999 ----------------------------------------------- All deaths in the population in 1999 Can multiple by 100 to get a percent.**Discussion Question 6**Why isn’t proportionate mortality a direct measure of risk?**Discussion Question 6**Because: The proportion of deaths from disease X tells us nothing about the frequency of deaths in the population ---- the overall risk of death in the population may be low.**OTHER MORTALITY MEASURES**Years of Potential Life Lost (YPLL): Measure of the loss of future productive years resulting from a specific cause of death. YPLL are highest when: • The cause of mortality is common or relatively common, AND • Deaths tends to occur at an early age.**Some Problems with Mortality Data**• Cause of death reporting from death certificates is notoriously unreliable • Changing criteria for disease definitions can make analyses over time problematic**Survival Analysis**•A technique to estimate the probability of “survival” (and also risk of disease) that takes into account incomplete subject follow-up. • Calculates risks over a time period with changing incidence rates. • Wide application in a variety of disciplines, such as engineering.**Survival Analysis**• With the Kaplan-Meier method (“product-limit method”), survival probabilities are calculated at each time interval in which an event occurs. • The cumulative survival over the entire follow-up period is derived from the product of all interval survival probabilities. • Cumulative incidence (risk) is the complement of cumulative survival.**K-M formula:**# of time intervals (Nk – Ak) S = ------------- k = 1 Nk Where: k = sequence of time interval Nk = number of subjects at risk Ak = number of outcome events**Survival Analysis**Example: • Assume a study of 10 subjects conducted over a 2-year period. • A total of 4 subjects die. • Another 2 subjects have incomplete follow-up (study withdrawal or late study entry). What is the probability of 2-year survival, and the corresponding risk of 2-year death?**Interpretation: Is the risk of death constant over**follow-up?**Survival Analysis**• With the Kaplan-Meier method, subjects with incomplete follow-up (FU) are “censored” at their last known time of (FU). • An important assumption (often not upheld) is that censoring is “non-informative” (survival experience of subjects censored is the same as those with complete FU). • Non-fatal outcomes can also be studied.**Survival Analysis**• The Life-Table method is conceptually similar to the Kaplan-Meier method. • The primary difference is that survival probabilities are determined at pre-determined intervals (i.e. years), rather than when events occur.