Feldmand. Module 1 – Intro to Epidemiology & Measures of Disease Flashcards
Definition of epidemiology
• From Greek
- – Epi (among, upon)
- – Demos (the people)
- – Logos (knowledge, doctrine)
• The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems (Dictionary of Epidemiology)
Goals of Epidemiology
- Elaboration of causes that explain patterns of disease occurrence
- Determine extent of disease
- Study natural history of disease
- To promote, protect and restore health
- Provide foundation for developing public policy and regulatory decisions
Typical Epidemiologic Approach
- Determine existence and magnitude of problem
- Describe WHO has the problem (animal/person, place, time)
- Develop hypotheses about WHY problem is happening
- Test the hypotheses using appropriate study designs and statistical tests
- Develop interventions based on findings
- Evaluate effectiveness of interventions
Basic Tenet of Epidemiology
• Disease does not occur randomly in a population
- – Disease occurrence is related to environment of species being studied
- – Environment includes physical, biological, sociological, meteorological, & management characteristics
• Epidemiology triad
Epidemiologic Triad
• Disease is result of forces within a dynamic interaction between
– Agent
– Host
– Environment
Measures of Disease: Ratios
- An expression of the relationship of 2 quantities
- Numerator is not in the denominator
- With a dimension – # of dogs owned / 100,000 population
- Without a dimension – # 2nd year vet students / # 3rd yr vet students
Measures of Disease: Proportions
- Ratio in which numerator is contained in the denominator
- Dimensionless
- Ranges from 0 – 1
- Tells us what fraction of population is affected
vets sitting ACVPM exam / Total # vets
Rates
- Ratio in which there is a relationship between the numerator and the denominator
- A true rate is instantaneous change in one quantity per unit change in another quantity (usually time).
- Tells us how fast disease occurs in a population
tests taken in vet curriculum / person-years in vet curriculum
Measures of Morbidity: Prevalence
• Prevalence (point prevalence, prevalence rate) – Proportion of pop’n with disease at a specific time
of subjects with disease at a point in time / Population at the same point in time
• Period prevalence – Frequency of disease for a given time interval
of subjects with disease for given time interval / Population at mid-interval
Interpretation of prevalence
Probability of having disease at a particular point in time.
Measures of Morbidity: Cumulative incidence, incidence proportion
– Proportion of subjects who develop disease during a certain time period
– Unitless, interpret in the context of time period
– Measure of average risk for a population
events during a period of time / Population without disease at beginning of period
Measures of Morbidity: Incidence • Incidence rate, incidence density
– Occurrence of new event per unit time
– The numerical value has no interpretability because it depends on the arbitrary selection of the time unit
new events / Total person-time at risk in population
Measures of Morbidity: Incidence • Attack rate
– Cumulative incidence used for particular populations observed for limited periods of time, as in an outbreak
– Usually expressed as a percent
events of dz during epidemic time period / Population at risk at start of period
Interpretation of cumulative incidence
Risk of developing disease over given time period
Interpretation of incidence density
Rapidity with which new new cases develop over given time period
Relation between Prevalence, Incidence & Duration
- Prevalence is not a measure of risk: Does not take into account the duration of the disease
- If prevalence (P) is small and incidence rate (I) and duration (D) are constant over time then P ≈ IxD
Measures of Mortality
- Mortality rate: Total # deaths from all causes in 1 year / Population at mid-year
- Disease-specific mortality rate: Total # deaths from specific dz in 1 year / Population at mid-year
- Case-Fatality Rate (CFR) # deaths after dz onset or diagnosis / # individuals with the dz
- Proportionate Mortality # deaths from specific dz in time period / Total # deaths in that time period
Mortality: when can it be a good index of the risk of disease?
• Can indicate severity of disease and also be an index of the risk of disease
- – If a disease is not typically fatal, mortality is not a good index of incidence • E.g., West Nile virus
- – When the case fatality rate is high and duration of disease is short, mortality is good reflection of risk • E.g., Rabies
Risk
- The probability of a disease-free individual developing a given disease over a specified period, conditional on that individual not dying from any other disease during the period
- Risk is without units, ranges from 0 to 1
- Risk=Attack rate in outbreak settings: # events of disease during time period / Population at risk at start of period
Odds
• The probability of an event occurring compared to the probability of that event not occurring
Probability event occurs / Probability event does NOT occur
2-by-2 tables
Disease Present Absent
Exposure +
Exposure -
Note that the disease status is across the top and the exposure status is on the left
(use of) Ratio Measures of Association
- Assess the strength or magnitude of the statistical association between the exposure and disease of interest
- In cohort studies, use relative risk
- In case-control studies, use odds ratio
Identification of Risk Factors
Epidemiological studies are conducted to identify risk factors through the comparison of incidence or prevalence between groups exposed and not exposed to a risk factor.
Probabilities of disease occurrence can be compared using:
- measures of strength of association: RR, OR
- measures of potential impact: attributable risk, attributable fraction.