Epidemiology for Public Health Practice MidTerm

concerned with the distribution (frequency of disease occurrence over a population not the individual) and determinants (factors that provoke a change) of health, diseases, morbidity, morality, and disability in populations.
Epidemiological Description
describes disease by age groups, time trends, geographic regions, other variables that affect distribution. This is different than the clinical approach which describes disease by symptoms.
factors that provoke a change in health. Ex. bio agents–bacteria. Chemical agents–carcinogens. Less specific—stress, high fat diet.
frequency of disease occurrence may vary from on population group to another.
Aims/Goals of Epidemiology
1. To describe health status of populations (to discover trends of disease occurrence).
2. To explain the etiology of disease to determine causal factors and mode of transmission.
3. To predict disease occurrence (number of cases to project appropriate mitigation measures).
4. To control the distribution of disease (the epidemiological approach is used to prevent the occurrence of new cases of disease, to eradicate existing cases, and prolong the lives living with disease.)
the occurrence in a community or region of cases of an illness (or an outbreak) clearly in excess expectancy.

an excessive occurrence (prevalence) of a disease. Pandemic refers to an epidemic on a global scale.

a disease that is habitually present in a particular geographical region.
Epidemic Threshold
Epidemic Threshold
the minimum number of cases that would support the conclusion that an epidemic is underway.
John Graunt
used birth and death data and established trend regularities.
Edward Jenner
Small pox vaccine
Natural Experiments
the epidemiologist observes changes in an outcome as the result of a naturally occurring situation.
John Snow
investigated London’s Cholera outbreak and used a natural experiment to find out.
William Farr
provided foundation for classification of disease (ICD system). Examined linkage between mortality rates and population density.
7 Uses of Epidemiology
1. To study the history of health populations.
2. to diagnose the health of the community.
3. to study the working of health service-operations research.
4. to estimate the individ. risk of disease.
5. to identify syndromes.
6. to complete the clinical picture of chronic disease.
7. to search for causes of health and disease.
Secular Trends
Secular Trends
(historical use) describes the changes in disease frequency over time.
Factors affecting reliability of Trends
1. lack of comparable data over time due to changing diagnostic criteria.
2. aging population
3. changes in the fatal course of the condition.
Disappearing Trend
no longer present in epidemic form. Ex: Small Pox
Residual Trend
good control not completely effective. Ex: STD’s
Persisting Trend
not effective treatment or prevention. Ex: Cancer, mental illness.
New Epidemic
increasing in frequency. Ex: AIDS, Obesity
Factors affection population size
births, deaths, and migration.
Demographic Transition
Due to general hygienic and social conditions. Decline in birth and death rate.
Epidemiological Transition
is followed by the demographical trans. and describes the shift of infectious disease to chronic diseases.
Risk Factor
exposure that is associated with a disease.
1. the freq. of disease varies by the value of the factor e.g., light smoker vs. heavy smoker.
2. it precedes the onset of the disease
Modern Causality
strength of the association
time sequence
consistency upon repetition
coherence of explanation
Biological Gradient
evidence of dose-response curve: that is an increase in disease risk in increase amount of exposure: Ex. smoking
if a association is biologically plausible than it will be helpful.
Experiment: Case-control and Cohort
experiment that compares people who have a disease (cases) with those who do not have it (controls).
People grouped based on variety of exposure and that are followed over time.
before agent reacts with host.
after agent reacts with host
Primary Prevention
Occurs during the period of prepathogenesis
Passive: does not require any behavioral changes. EX: Fluoridation of water.
Active: requires behavioral change. Ex: Vaccinations, smoking cessation.
2nd Prevention
Occurs during the pathogenesis phase and includes early diagnosis, prompt treatment, disability limitations.
Tertiary Prevention
occurs during the late phase of pathogenesis and includes rehab to restore the patient to functional level.
the number of cases of a disease
the value obtained by dividing one quantity over another.
Include: rates, proportions, and percentages.
the numerator is part of the denominator and could be expressed as a percentage.
a/a+b X 100=
a ratio that consists of the numerator and the denominator in which time is part of the denominator.
Crude death rate: # of deaths in a year/reference population (midpoint)
and use a multiplier of 1,000 or 100,000
the number of existing cases of a disease at some designated time.
the rate of development of a disease in a group over a time period.
1. numerator: the number of new cases
2. denominator: the population at risk
Attack rate
different form incidence rate and is often used over a short period of time in response to an outbreak.
ill/(ill+well) x 100 (during a time period)
Interrelationship of P and I
the P of a disease is proportional to the I rate x the duration D of a disease.
Specific Rate
a subgroup of the population that defined in terms of sex, race, age, or single cause of death.
Adjusted Rate
rate of morbidity or mortality in which statistical mechanisms have been applied to allow fair comparisons across the population by removing the the effect of differences in composition of various populations.
Indirect age-adjusted
A=age group
B= total estimated population
C=death rate in standard population
D=expected number of death in interest population
D= (total estimated population(B) * death rate in standard population(C)) / 100,000
Direct age-adjusted
Need to determine rate first (C)
((A) Deaths/ (B) Population)*100,000 =C
(C) rate* (D) standard population/100,000=age-adjusted for death
Why is age-adjustment important?
allows the investigator to make fairer comparisons between different age groups. For instance, the older populations might have higher frequencies of disease simply because they are older which could lead to misinterpretation of data.
Direct Adjusted Rate
used if age-specific death rates in a population to be standardized are known and a suitable standard population is available.
Indirect Adjusted Rate
If age-specific death rates of the population for standardization are unknown or unstable, for example, because the rates to be standardized are based on a small population.
Use SMR (Standard Mortality ratio)
Standard Mortality Ration (SMR)
observed deaths/expected deaths x100
Interpretation of SMR
if observed and expected number are the same: SMR =1.0 or 100%
SMR = 2.0 than the death rate is two times greater than the expected.
Less, then the observed are fewer. More, the observed is greater.
Descriptive Studies
used to identify a health problem that may exist. Characterize the amount and distribution of disease.
Analytic studies
follow descriptive studies, and are used to identify the cause of the health problem.
Positive declaration
Research hypothesis. Ex: the infant mortality is higher in one region that another.
Negative Declaration
Null hypothesis. Ex: there is NO difference between the infant mortality rates of two regions.
Implicit Question
to study the association of….whatever.
The method of difference
all the factors are the same expect for a single factor. Thus, the variable. Ex: the study of the role of physical activity in reducing morbidity of CHD.
The method of agreement
a single factor is common to a variety of different settings. Ex: wherever there is air pollution there is an increase in COPD.
The method of concomitant variation
frequency of disease corresponds to dose.Ex: smoking, the more one smokes the greater the risk.
The method of residues
subtracting potential causal factors to find factor(s) with greatest impact. Ex: multiple regression analysis used to determine the number of risk factors associated with CHD.
The method of analogy
The mode of transmission and symptoms of a disease of unknown etiology bear a pattern similar to that of a known disease.
infant mortality
used for international comparisons; a high rate indicates unmet health needs and poor environmental conditions.
3 Approaches of Des. Epi
Case Reports (cases)
Case series (summarize the characteristics of patients from a clinical settings.)
Cross-sectional studies. ( surveys of the population to estimate the prevalence of a disease or exposure.
The importance of characteristics of person, place or time
person, place or time can directly or indirectly affect the occurrence of disease because they affect a wide variety of exposures associated with lifestyle, behavioral patterns, access to medical care and exposure to environmental hazards.
descriptive studies
are concerned with characterizing the amount and distribution of diseases in a population.
analytical studies
are concerned with determinants of disease, reasons for high or low frequency of disease in a specific subpopulation
Ch of Person: Age
most important when describing age because age-specific disease rates show greater variation than rates defined by any other personal attribute.
4 reasons for age associations
1. the validity of the diagnosis: affected by classification errors.
2. Multimodal Trends: meaning that there are peaks and declines in the frequency of diseases at different ages.
3. Latency effects: between environmental exposures and and the development of the disease. Ex. Cancer
4. human biological clock: immunity wanes and increased cell division.
Ch of person: marital status
married people have lower rates of morbidity and mortality.
marital status hypothesis
Protective: may provide an environment that is conducive to health.
Selective: people who marry may be healthier to begin with.
health migrant effect
observation that healthier individuals, typically younger persons form the majority of migrants.
Socioeconomic status
excess mortality and morbidity. Factors include poor housing, crowdedness, low income, racial disadvantage, poor education, etc.
Measurement of Social Class
Prestige of occupation, educational attainment, income.
Social causation (breeder hypothesis)
conditions arising from membership of low social class groups produce mental illnesses.
downward drift hypothesis
the clustering of psychosis will drift down to lower classes.
Ch of Place
Internationally: Differences in infectious disease, climate, culture. WHO tracks this.
Geographic: variation within a country.
Urban/rural difference: urban diseases are due to overcrowding pollution and poverty.
Localized occurrence of disease.
person-to-person diseases found in urban areas.
Geographical Information Systems (GIS)
a method to provide a spatial perspective on the geographical distribution of health conditions.
Reasons for place variation in disease
Gene/environment interactions.
Influence of climate
environmental factors: ex; chemical agents linked to cancer.
Ch of time
cyclic fluations
point epidemics
secular time trends
cyclic fluctuations
more accidents in the summer and flu in the winter
unusual aggregation of health events grouped together in time and space.
Temporal: post vaccinations
Spatial: concentration of a disease in a specific geographical area.
Secular time trends
gradual changes in frequency of a disease over long periods of time.
Point epidemic
the response to a group of people circumscribed in a place and time to a common source=of infection or contamination and exposed simultaneously.
Quality and use of Epi data
nature of data
availability of data
completeness of population coverage
strengths vs. limitations
Nature of data
the type of data: vital stats, cases, etc.
makes conclusions about inferences that can be made.
availability of data
the investigator’s access to data
Completeness of population coverage
Representativeness: aka external validity: refers to the generalizability of findings to the population from which the data have been taken.
Thoroughness: how detailed is it.
Strengths vs. limitations
the utility of data for various types of epi research.
Pres. #1: Assessment of 2010 global measles
the WHO endorsed a goal of reducing measles mortality by 90% over the course of 10 years. The conclusion was that this goals was not met due to India and outbreaks in Africa.
MCV1=measles containing vaccine.
Pres. #2: Mortality and Hospital stays with MRSA and EC
purpose was to estimate the impact of morbidity mortality and demands on health care services of antibiotic resistant strains of MRSA and EC.
Concluded that there was excess mortality with BSI’s caused by MRSA and Ec, thus prolonging hospital stays and imposes a burden on the healthcare system.
Pres. #3: Sport-related brain injury
the purpose was to determine the incidence, nature and severity of sport-related injuries in the general population.
Pres.#4: Advanced Parental Age and the Risk of Autism Spectrum Disorder
Purpose was to evaluate the effects of maternal and parental age on the risk of autism spectrum disorder.
It used a cohort case design.
Concluded that ASD risk increases with both maternal and parental age and decreases with birth order.
Pres. #5: adverse effects in young adult reporting use of energy drinks and caffeine
Purpose was to assess the prevalence of psychological and behavioral adverse effects among young adults in ER patients who reported drinking caffeine or energy drinks prior to ER admission.
Concluded that those who consumed energy drinks had higher report of adverse effects compared to those who only drank caffeine drinks.
descriptive studies
cross-sectional surveys used to depict and individual’s health characteristics with respect to time place and person.
Hypothesis generation
analytical studies
ecological studies, case-control studies, cohort studies. used to test specific etiological hypothesis.
Hypothesis testing
Experimental study
has the greatest control of the research study by which the investigator can manipulate the subject and the variable.
Ecologic study
examines a group as the unit of analysis insofar as other studies use the individual as the unit of analysis.
Ex: a study of mortality from lung disease in different cities that are known to have differing levels of air pollution would comprise a ecologic study. The unit of analysis is the city.
Limitations: the ecologic fallacy
Important: the level of exposure for each individual in the unit being studied is unknown.
Ecologic comparison study
examines exposure and disease rates among different groups over the same time period.
Ecologic trend study
examines the correlation of changes in exposure and disease within the same community, country or other aggregate unit.
Ecologic Fallacy
Observations made at the group level. may not represent the exposure-disease relationship at the individual level.
The ecologic fallacy occurs when incorrect inferences about the individual are made from group level data.
Cross-sectional study
a type of descriptive study designed to estimate the prevalence of a disease or exposure. AKA prevalnce study
less time consuming that case-control or cohort studies
good, quick picture of prevalence of exposure and prevalence of outcome
difficult to determine the temporal relationship between exposure and outcome (lacks the element of time)
may have excess prevalence from long duration cases (such as cases that last longer than usual but may not be serious).
uses of cross-sectional study
Hypothesis generation
Intervention planning
Estimation of the magnitude and distribution of a health problem
Limitations: because of a relative lack of utility for studies of disease etiology.
Does not provide incidence data.
Cannot study low prevalence of diseases (e.g., mental illness)
Cannot determine temporality of exposure/disease.
Case-control study
a type of analytical study that compares individuals who have a disease with individuals who do not have the disease in order to examine differences in exposures or risk factors for the disease.
relatively inexpensive.
less-time consuming than cohort studies.
can evaluate effects of multiple exposures.
efficient for rare outcomes or outcomes with long induction or latency periods.
subject to recall bias (based on subject’s memory and reports).
inefficient for rare exposures.
difficult to establish clear chronology of outcome and exposure.
Interpretation of OR (odds ratio)
OR=1.0: the odds of exposure are equal among the cases and controls thus, is not a risk factor therefore no association.
OR=2.0: suggests that the cases were twice as likely as the controls to be exposed.
OR= <1.0: suggests it is a protective factor.
a population group that is followed over a period of time
Cohort Studies
a type of analytical study that collects data and follows a group of subjects who received a specific exposure. The incidence of a disease or outcome of interest is tracked overtime. The incidence in the exposed group is compared with the incidence in groups that are not exposed, have different levels of exposure, or have different types of exposures.
Can evaluate multiple effects on a single exposure.
More efficient rare exposures and outcomes with long induction or latency periods.
Can directly measure incidence.
Clear chronological relationship between exposure and outcome.
direct determination of risk
can study multiple outcomes
can study rare exposures
Inefficient for rare outcomes with long induction or latency periods.
Subjects lost over time thus no more follow-ups.
Population Based Cohort
the cohort includes the entire population or a representative sample of the population.
Framingham Study:
Cohort study of CHD
exposure=unknown until 1st period of observation
Can be generalized over the entire population
not efficient for rare exposures.
Exposure Based Cohort
subjects with a common exposure (occupational)
Ex. Workers exposed to lead paint
Weakness=rare exposure
types of temporal cohort studies
prospective, retrospective and historical prospective
prospective cohort study
determines levels of exposure at baseline (present) and follow up for occurrence of disease at some time in the future.
advantages of prospective cohort studies
enables to collect data on exposures
the size is under control of the investigator
bio and physio assays can be done with less concern of the outcome being affected by the course of the disease
direct measures on the environment
Retrospective cohort study
use historical data to determine the level of exposure at some baseline from the past
advantages of retrospective cohort studies
do not have to wait for follow-up collections of data with saves time.
Its cheaper to use the data that is already collected and possibly could be quite extensive data.
cohort study: measure of association-relative risk
direct measure of association of exposure and outcome
relative risk
is the ratio of the incidence of the disease in the exposed group to the incidence of the non-exposed group.
nested case-control study
type of case-control study in which cases and controls are drawn from the population in a cohort study
that population then comprises both exposed and non-exposed persons.
advantages of nested case-control studies
have control over confounding factors
reduced cost b/c exposure information is collected from a subset of the cohort only
e.g., suicide among electricity workers
strengths of cohort studies
direct determination of risk
timing sequence of exposure and outcome
can study multiple outcomes
can study rare exposures
weaknesses of cohort studies
takes a long time
subjects lost to follow-ups
Clinical trial
a carefully designed experimental investigation of the effects of a treatment or technology that uses randomization, blinding of subjects to study conditions, and manipulation of the study factor.
ecologic study
a type of descriptive study that assesses the correlation (association) between exposure rates and disease rates among different groups or populations over the same time period. The unit of analysis is the group.
Ex: a study of mortality from lung disease in different cities that are known to have different levels of air pollution would comprise an ecologic study. The city is the unit of analysis.
Less-time consuming.
simple and easy to understand.
examines community, group, or national level of data or trends.
generating hypothesis
ecologic fallacy, which infers association of the the population level.
Difficult to detect complicated exposure-outcome relationships.