epi511 set 1
Terms
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- Epidemiology
- Study of distribution and determinants of health and disease in POPULATIONS
- Primary prevention
- prevent disease from occuring
- Secondary prevention
- early detection to change natural history
- Tertiary prevention
- reducing disability after treatment
- Determinants of distribution
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Infectious agents
Carcinogens, chemicals
Behaviors, habits, lifestyle
Occupational exposures
aging
genetic constitution
social/political factors - Epidemiologic triad includes?????
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Host
Environment
Agent - Distribution:
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Who
Where
When
How
Changes - Epidemic?
- Higher than normal levels of disease
- John Snow used what approach to epidemiology?
- Ecologic and observation or retrospective cohort design
- Koch's postulates
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1. agent recovered from all diseased individuals
2. Agent grows in culture
3. Cultured organism causes new disease in susceptible
4. Agent recovered from new diseased animal - Revised causal criteria for disease
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1.Temporal relationship
2.Strength of association (relative rate)
3. Biologic plausibility
4. Risk factor
5. Consistency: association is replicated by other investigators
6. Dose response - Herd immunity
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1. Resistance to the spread of infectious disease in a group because susceptible members are few, making transmission from an infected member unlikely.
2. The immunologic status of a population, determined by the ratio of resistant to susceptible members and their distribution. - Outbreak investigation
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1. define numerator (cases)
a. case definition
2. define denominator: population at risk
3. Calculate attack rate (incidence rate of people at risk) - Attack rate:
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# at risk with disease/total # at risk
(%) - Secondary attack rate:
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# contacts who develope disease/
#total susceptible contacts
this also is a measure of tendency of spread in a population - Attack rate ratio:
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Attack rate % for sick students who ate fish /Attack rate % for sick students who did not eat fish.
A good rule of thumb for determining a relationship for attack rate ratios (or risk ratios) in foodborne disease:
>5 Very strong association (increased risk)
3.0 -<5.0 Strong association (increased risk)
1.7 -<3.0 Moderate association (increased risk)
1.3-1.6 Weak association (increased risk)
0.9-1.2 Probably no association
0.5-0.8 Weak association (decreased risk) - Incubation Period
- Interval from recipt of infection to the time of onset of clinical illness
- susceptibile
- the probability to get disease (NEVER 0)
- non-susceptible
- the probabilty to get disease IS ZERO
- Steps to epidemiology study
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1. Define disease
2. Define population
3. Find all cases in the population (existing and new)
4. Create measures of case frequency per population - Prevalence (A SNAPSHOT)
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# the total number of cases of a given disease in a specified population at a specified time and/or
# the ratio of the number of cases of a disease present in a statistical population at a specified time and the number of individuals in the population at that specified time.
NOT a measure of risk and NOT a rate - Point prevalence
- How many on a certain date? aka cross-sectional sample
- Period prevalence
- disease cases present during a specific time interval (NOT a good measure)
- prevalence case bias
- bias due to longer disease survivability
- RATES
- proportions, ratios, risk and instataneous rates over TIME
- Proportion
- includes the numerator in the denominator
- ratio
- numerator and denominator come from different groups (male/female)
- Risk
- result of rates that prevail over a period of time. Time is not a dimension; only used descriptively to specify a period of observation
- Cumulative incidence CI:
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number of new cases divided by the candidate population over a period of time.
also be calculated by the incidence rate multiplied by duration.
CI(t)=1-e^{-IR(t) * D}
new cases/initial population at risk - Incidence Rate IR (density)
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new cases/at risk time
"stroke incidence for males is 5 cases/100,000 person-years - Incidence Density ID
- new cases/person-years
- Crude rates
- total number of something per 1000 people. summary for a population of comparison age group. Not used for inter-population comparisons.
- standardised mortality rate (SMR) or age-specific mortality rate (ASMR)
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total number of deaths per 1000 people of a given age (e.g. 16-65 or 65+).
Factors affecting a country's death rate
* Nutrition levels
* Standards of diet and housing
* Access to clean drinking water
* Hygiene levels
* Levels of infectious diseases - Specific Rates
- ALWAYS can be compared between groups.
- Direct Standardization of rates
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1. choose a standard population
2. multiply specific rates from pop#1 by standard pop age groups
3. sum the pop#1 and divide by total standard pop, then repeat for #2.
ADJUSTS FOR CONFOUNDING EFFECT - Protortionate Mortality Ratio (PMR)
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observed deaths in population A/expected deaths based on the proportion in the population B.
Used when actual population numbers are not known. NOTE: NOT rates or risks - Crude Birth rate
- #live births in a year/average midyear population x 1000
- crude death rate
- #deaths in a year/ave mid-year pop x 1000
- Age-specific death rate
- #deaths in certain age in a year/ave pop in age group x 100
- cause-specific death rate
- #deaths from cause/mid-year pop x 1000