Upload
amaya-goodwin
View
25
Download
0
Embed Size (px)
DESCRIPTION
What is Epidemiology?. Lilienfeld A: in Foundations of Epidemiology THE STUDY OF THE DISTRIBUTION OF A DISEASE OR A PHYSIOLOGICAL CONDITION IN HUMAN POPULATIONS AND OF THE FACTORS THAT INFLUENCE THIS DISTRIBUTION - PowerPoint PPT Presentation
Citation preview
What is Epidemiology?What is Epidemiology?
Definitions of EpidemiologyDefinitions of Epidemiology
Lilienfeld A: in Foundations of Epidemiology
THE STUDY OF THE DISTRIBUTION OF A DISEASE OR A PHYSIOLOGICAL CONDITION IN HUMAN POPULATIONS AND OF THE FACTORS THAT INFLUENCE THIS DISTRIBUTION
(Note that epidemiology is applied to a broad variety of health related issues – not just infectious diseases.)
Purposes Of EpidemiologyPurposes Of Epidemiology
1. Identify causes and risk factors for disease.
2. Determine the extent of disease in the community.
3. Study natural history and prognosis of disease.
4. Evaluate preventive and therapeutic measures
5. Provide foundation for public policy
What Is The Unique Skill What Is The Unique Skill Of Epidemiologists?Of Epidemiologists?
MEASURING DISEASE FREQUENCY IN
POPULATIONS
Measuring Disease Frequency Measuring Disease Frequency Has Several ComponentsHas Several Components
Classifying the disease
Deciding what constitutes a case of disease in a study
Finding a source for ascertaining the cases
Defining the population at risk of disease
Defining the period of time of risk of disease
Obtaining permission to study people
Making measurements of disease frequency
Relating cases to
population and time at risk
Case DefinitionCase Definitioncase definition -defines a case in time, person and
place. Time criteria may include all cases of a disease identified from, for
example, January 1, 2008 to March 1, 2008. Person criteria may include age, gender, ethnicity, and clinical
characteristics such as symptoms (e.g. cough and fever), clinical tests (e.g. pneumonia on chest X-ray).
Place criteria will usually include a geographical entity such as a town, state, or country but may be as small as an institution, a school class, or a restaurant meal session.
Two Broad Types of Two Broad Types of EpidemiologyEpidemiology
(Who? Where? When?) Examining the distribution of a disease in a population, and observing the basic features of its distribution in terms of person, place, and time.
Typical study design:
community health survey (approximate synonyms - cross-
sectional study, descriptive study)
(How? Why?) Testing a specific hypothesis
about the relationship of a disease to a putative cause, by conducting an epidemiologic study that relates the exposure of interest to the disease of interest.
Typical study designs:
-cohort study -case-control study
DESCRIPTIVE EPIDEMIOLOGY
ANALYTIC EPIDEMIOLOGY
Cross Sectional StudyCross Sectional Study
Basically this is a survey– Defines populations to be studied– Collects information from pop. members about
their disease and exposure– “snapshot” of the population – point in time
– Examines relationship between a variable and a disease – but does NOT establish cause and effect which requires data over time
The Basic Triad Of The Basic Triad Of
Descriptive EpidemiologyDescriptive Epidemiology THE THREE ESSENTIAL CHARACTERISTICS OF DISEASE WE LOOK
FOR IN DESCRIPTIVE EPIDEMIOLOGY:
TIME PLACE PERSON
Time Person
Place
TimeTime
Changing or stable?
Seasonal variation.
Clustered (epidemic) or evenly distributed (endemic)?
Point source or propagated.
PlacePlace
Geographically restricted or widespread (pandemic)?
Relation to water or food supply.
Multiple clusters or one?
PersonPersonAge
Socio-economic status
Gender
Ethnicity/Race
Behavior
Descriptive Epidemiology Is A Descriptive Epidemiology Is A Necessary Antecedent Of Necessary Antecedent Of
Analytic EpidemiologyAnalytic Epidemiology To undertake an analytic
epidemiologic study you must first: Know where to look Know what to control for Be able to formulate hypotheses
compatible with laboratory evidence
The Basic Triad Of The Basic Triad Of Analytic EpidemiologyAnalytic Epidemiology
THE THREE PHENOMENA ASSESSED IN ANALYTIC EPIDEMIOLOGY ARE:
HOST
ENVIRONMENTAGENT
AgentsAgents Microbes = Classic view – others include:
Nutrients – (ex rickets)
Poisons Allergens Radiation Physical trauma (car crashes)
Psychological experiences
Host FactorsHost Factors
Genetics
Immunologic state
Age
Personal behavior
EnvironmentEnvironmentInfluences interaction of host and agent Crowding Atmosphere/weather Modes of communication – phenomena
in the environment that bring host and agent together, such as:– Vector– Vehicle– Reservoir
Public Health Strategies…Public Health Strategies…
Depending on which public health strategy might be most effective, we might direct efforts at the specific agent (guinea worm), host ( immunization to prevent measles), or environment (sanitation improvements to prevent salmonella)
Analytical studies:Analytical studies:
Cohort Case Control
Cohort StudyCohort Study
Follows population forward in time from suspected cause to effect
Begins with a group free of Disease.– Selects population according to exposure – Not
whether they have the disease or health problem or not. Population is WELL DEFINED
See if the development of new cases differs between exposed and unexposed groups
Uses Relative Risk Calculation.
Cohort Study Cohort Study Example: A salmonella outbreak occurs at a Example: A salmonella outbreak occurs at a
company picnic….company picnic….Each attendee is questioned about the food
they ate (exposure) and whether they became ill afterward.
Relative risk for exposure can be CALCULATED (quantified) from the relationship between the exposure and outcome.
Relative Risk calculator at Medcalc/org
Case Control StudyCase Control StudyWork backwards from Effect Cause
Case subjects that have the disease are – compared to Control subjects who do not have the
disease but are part of the same at-risk population. Population is not well defined as in Cohort studies
You cannot calculate attack rates because you do not know the total number of people in the community who were and were not exposed to the source of the disease under study. Without attack rates, you cannot calculate relative risk
Comparison of the two groups is quantified by an ODDS RATIO*:
*See below in notes::.
Odds Ratio Calculator (Medcalc.org) The odds ratio is used frequently in
case/control studies • In a case/control study, ill persons’ (“cases”)
characteristics and exposures are compared with those of well persons (“controls”) selected from the same population in which the outbreak occurred.
– Example:
– in an outbreak suspected to stem from exposure to contaminated food at a restaurant, the ill persons’ food selections could be compared with those of some well people who also ate at the restaurant the same day. This could be done even if we didn’t know exactly how many people ate at the restaurant that day.
• A two-by-two table is constructed – The OR is calculated by multiplying across the
cells.
The formula for OR is: The formula for OR is: Odds Ratio = Odds Ratio = ad ad bc bc Where
a = number of persons with disease and with exposure of interest
b = number of persons without disease, but with exposure of interest
c = number of persons with disease, but without exposure of interest
d = number of persons without disease and without exposure of interest
a+c = total number of persons with disease (“cases”)
b+d = total number of persons without disease (“controls”)
Number of Cases of Disease X by Exposure Number of Cases of Disease X by Exposure
History, Smallville, 2004 Disease X History, Smallville, 2004 Disease X
Yes No Total
Ate Tuna Casserole a =46 b= 25 71 Didn’t Eat Tuna c= 18 d= 40 58
CasseroleThe OR in this example is: Odds Ratio = 46 x 40 = 1840 = 4.1
25 x 18 450 So those who became ill were 4.1 times as likely to have eaten the tuna
casserole. We should probably look a little more deeply into the tuna casserole!
Men Women Children Total
1st class 67% 3% 0 38%
2nd class 92% 14% 0 59%
3rd class 84% 54% 66% 62%
Total 82% 26% 48% 62%
EVERY HEALTH OUTCOME HAS SOME INTERESTING AND USEFUL EPIDEMIOLOGIC
CHARACTERISTIC
DEATH RATES BY SOCIAL CLASS FROM A CERTAIN CAUSE AMONG 1,316 PEOPLE
WHAT CAUSE OF DEATH IS THIS?
The previous slide shows death The previous slide shows death rates by class of ticket on the rates by class of ticket on the
Titanic, a large ocean liner that Titanic, a large ocean liner that sank after colliding with an sank after colliding with an
iceberg in 1912 iceberg in 1912