Epidemiology

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Incidence Rate

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measures the rapidity with which new cases are occurring.
IR=# of new cases/time disease free individuals are observed

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Prevalence

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PR=# of existing cases/# of people in population at that time

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Cumulative Incidence (Incidence Proportion)

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Measures Frequency of addition of new cases (always counted for a given time), most common way to estimate risk, assumes a fixed cohort, for brief periods of time.
best for fixed population
Proportion
CI (IP)=# of new cases/# of disease free subjects at start

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Period Prevalence

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Measures the frequency of disease with a period of time
PP=# of prevalent cases at beginning of time + #of new cases that develop/size of population

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Point Prevalence

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measures the frequency of a disease at a given point in time
PP= # of observed cases a time/population size at the time

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Limitations of Prevalence and incidence

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-Defining who has the disease
-Ascertaining which persons should be included in numerator.
-Selective under counting of certain groups in population may occur.

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Uses of Prevalence

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-Good for determining burden of disease
-Helpful to determine how to allocate resources
-Good for planning health services

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Uses of Incidence

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-Good for forecasting and monitoring epidemics
-Good for evaluation pf primary prevention programs
-Measurement of risk
-Good for studying etiology

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Incidence Density

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useful when individuals are observed for different lengths of time and can be used for dynamic populations
True rate

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Crude Mortality Rate

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CMR=(deaths occurring during a given time person/# in population at midyear) x per __ population

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Case Fatality Rate

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CFR=# of people dying during a specified period/# of people with specified disease)

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Cause Specific Mortality Rate

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CSMR= # of deaths from a specific disease.# of people in population at midyear

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Attack Rate

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AR= # of people who are ill/# in population at risk

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Proportional Mortality Ratio

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PMR=# of deaths from a specific disease/total deaths from all causes.

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Epidemiology Objectives

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-identify the etiology of a disease and relevant risk factors.
-determine extent of disease
-study natural history and prognosis of disease
-evaluate preventative and therapeutic measures and modes of health care delivery
-provide the foundation for developing public health

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Characteristics of Epidemiology

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-Population focused
-distribution
-determinants
-outcomes
-quantification
-control of health problems

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Descriptive Epidemiology

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amount and distribution of health and disease within a population

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Analytic Epidemiology

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examines causal hypotheses regarding the associations between exposure and health conditions

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William Farr

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developed more sophisticated system for codifying medical conditions and examined linkage between mortality rates and population density.

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John Snow

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investigated cholera outbreak and proposed it was transmitted through contamination and was able to demonstrate association by moving water. recommended a public health measure to prevent disease

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Epidemiologic Transition

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shift in patterns of mortality from infectious and communicable disease to chronic and degenerative diseases

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Demographic Transition

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shift in increased birth and death rates to a low birth and death rate.

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Hills guidelines of casualty

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-Strength of association (the stronger the association the more likely it is a causal relationship)
-consistency (observe it consistently in different studies and populations)
-Specificity (certain exposure should lead to a single outcome)
-temporality (cause must come before effect)
-biological gradient (as dose of exposure increase so does risk of the disease)
-plausibility (should be an existing biological or social model to explain the association
-coherence (shouldn't seriously conflict with the generally known facts of natural history and biology of the disease)
-experiment (intervention that modifies exposure should result in less or no disease at all)
-analogy (similar relationship has been observed with another exposure and/or disease)

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Ecological Fallacy

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we attribute to members of a group, the characteristics of the group which they in fact do not posses as individuals

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Ratio

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any value that has a numerator and denominator with no specified relationship includes rates and proportions

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Rate

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measures how quickly something happens
denominator includes a measure of time.

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Proportions

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a ration in which the numerator is part of the denominator. always between 0 and 1 and expressed as a percentage.

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RR>1

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[(RR-1)x100] risk in exposed is greater than risk in unexposed

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RR=1

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No association

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RR<1

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[(1-RR)x100] risk in exposed is less than risk in unexposed

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RR>2

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how many time more likely to develop the disease

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RR=0

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no excess cases in exposed

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Odds Ratio

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OR=(a/b)/(c/d)

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Cumulative Incidence Ratio

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CIR=[a/(a+b)]/[c/(c+d)]