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 Epidemiology
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Classically speaking

• Epi upon (among)
• Demos people
• Ology science
• Epidemiology the science which deals with what
  falls upon people..

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A Modern Definition

• The study of the distribution and determinants
  of health-related states in specified
  populations, and the application of this study to
  control health problems."
• (Last J)

Search for knowledge Apply in health
service
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Objectives of Epidemiology

• 1. To describe the distribution and magnitude of
  health and disease problems in the population.
• 2. To identify the etiological factors risk
  factors in the population.
• 3. To provide the data essential to planning,
  implementation and evaluation of services for
  prevention, control and treatment of disease and
  to setting up of priorities for these services.

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• The ultimate aim of epidemiology is
• to eliminate or reduce health problem or its
  consequences
• and
• to promote health and well-being of society as a
  whole.

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Purposes of Epidemiology

• To investigate nature / extent of health-related
  phenomena in the community / identify priorities
• To study natural history and prognosis of
  health-related problems
• To identify causes and risk factors
• To recommend / assist in application of /
  evaluate best interventions (preventive and
  therapeutic measures)
• To provide foundation for public policy

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Component

• Disease Frequency- Rate and Ratio e.g
• Rate- incidence rate,
  prevalence rate etc
• Ratio- sex ratio, doctor-population ratio
• Distribution of Disease-
• Disease in community find causative
  factor
• Generate hypothesis
• Descriptive epidimiology

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• Determinants of Disease-
• To test hypothesis
  
• Analytic epidemiology
  
• Help in
  develop sound scientific
  program

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Incidence

• Number of new cases of a disease which come into
  being during a specified period of time.
• (Number of new cases of specific disease
  during a given period)/(population at risk during
  that period) x 1000
• Importance If incidence increasing, it may
  indicate failure or ineffectiveness of control
  measure of a disease and need for better/new
  health control measure.

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Prevalence

• Number of current case (old and new) of a
  specified disease at a point of time
• It help to estimate the burden of disease
• Identify potentially high-risk populations. They
  are essentially helpful to plan rehabilitation
  facilities, manpower needs, etc.
• (Number of current case of a specified disease
  at a point of time)/(estimated population at the
  same point of time) x 100
• Point prevalence AND Period prevalence

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Relationship between incidence and prevalence

• Prevalence Incidence x Duration

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Approach of an epidemiologist

• Asking questions
• making comparisons
• ? Asking questions may provide clues to cause or
  aetiology of disease e.g.
• What is the event,
• what is its magnitude,
• where did it happen,
• when did it happen,
• who were affected,
• why did it happen?

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• Making comparisons will help draw inferences to
  support asking questions.
• This comparison may be
• ? Between those with the disease and those
  without the disease
• ? Those with risk factor and those not exposed to
  risk factor

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Terms to know

• Endemic constant presence of a disease in a
  given population
• epidemic outbreak or occurrence of one specific
  disease from a single source, in a group
  population, community, or geographical area, in
  excess of the usual level of expectancy
• pandemic epidemic that is widespread across a
  country, or large population, possible worldwide

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Epidemiology versus clinical medicine

• Epidemiology

• Clinical medicine

• Unit of study is case
• Concerned with only sick
• Patient comes to doctor
• Seeks diagnosis, derives prognosis, prescribes
  specific treatment

• Unit of study is a defined population or
  population at risk
• Concerned with sick as well as healthy
• Investigator goes to the community
• identify source of infection, mode of spread, an
  Etiological factor, future trend or recommend
  control measures

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Three essential characteristics that are examined
to study the cause(s) for disease in analytic
epidemiology are...

• Host
• Agent
• Environment

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Host Factors

• Behaviors
• Genetic predisposition
• Immunologic factors

• Influence the chance for disease or its severity

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Agents

• Biological
• Physical
• Chemical

• Necessary for disease to occur

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Environment

• External conditions

• Contribute to the disease process

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Epidemics arise when host, agent, and
environmental factors are not in balance

• Due to new agent
• Due to change in existing agent (infectivity,
  pathogenicity, virulence)
• Due to change in number of susceptibles in the
  population
• Due to environmental changes that affect
  transmission of the agent or growth of the agent

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EPIDEMIOLOGICAL METHODS

• .
• The methods he employs can be classified as
• 1. Observational studies
• a. Descriptive studies
• b. Analytical studies
• Case control studies
• Cohort studies
• 2. Experimental/interventional studies
• Randomized control studies
• Field trials
• Community trials

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• Descriptive observations pertain to the who,
  what, where and when of health-related state
  occurrence. However, analytical observations
  deal more with the how of a health-related
  event occur.
• Randomized control trial (often used for new
  medicine or drug testing), field trial (conducted
  on those at a high risk of conducting a disease),
  and community trial (research on social
  originating diseases)

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• Descriptive Studies
• Steps in conducting a descriptive study.
• Descriptive studies form the first step in
  any process of investigation.
• These studies are concerned with observing
  the distribution of disease in populations.
• 1. Defining the population.
• 2. Defining disease under study.
• 3. Describing the disease.
• 4. Measurement of disease
• 5. Compare
• 6. Formulate hypothesis-

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• Defining the population. Defined population may
  be the whole population or a representative
  sample.
• It can also be specially selected group such as
  age and sex groups, occupational groups, hospital
  patients, school children, small community, etc.

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• 2. Defining disease under study.
• 3. Describing the disease.
• Disease is examined by the epidemiologist by
  asking three questions
• ? When is the disease occurringtime
  distribution?
• ? Where is it occurringplace distribution?
• ? Who is getting the diseaseperson distribution?

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• A. Time Distribution
• Short-term fluctuations.
• Common source epidemics
• - single exposure/point sourcebhopal
  tragedy
• Propagated-infectious Hep A
• Periodic fluctuations
• Seasonal measles (early spring)
• cyclic- ,, in pre-vaccinated
  era (peak 2-3 yr)
• Long-term or secular trends diabetes, CVD

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• B. Place Distribution
• International variations
• Cancer of stomach very common in Japan
• less common in US.
• oral cancer- India
• Breast cancer- Low-japan, high-western
• National variations, e.g. Distribution of
  fluorosis,

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• Rural-urban differences, e.g.
• CVD, Mental illness more common in urban areas.
• Skin diseases, worm infestations more common in
  rural areas.
• Local distributions, e.g. Spot maps- John Snow in
  London to incriminate water supply as cause of
  cholera transmission in London.

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cholera cases in proximity to water pump, 1854
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• C. Person Distribution
• Age e.g.
• Measles is common in children,
• Cancer in middle age
• Degenerative diseases in old age.
• Sex
• Women- Lung cancer-less
• Hyperthyroidism- more
• c. Social class- Diabetes, Hypertenson upper
  class

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• 4. Measurement of disease- Mortality/ Morbidity
• 5. Compare- Between different population,
  subgroups
• 6. Formulate hypothesis. On basis of all data
  epidemiologist form hypothesis.

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Cross-sectional studies

• Cross-sectional study is also called prevalence
  study.
• Cross-sectional study is the simplest form of
  observational study.
• It is based on single examination of
  cross-section of population at one point of time.
  
• If the sampling methodology is accurate, results
  can be projected to the entire population.
• They are more useful for chronic illnesses, e.g.
  hypertension.
• Cross-sectional studies save on time and
  resources, but provide very little information
  about natural history of disease and incidence of
  illness.

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Case- control studies

• It start from effect and then proceed to cause
• Both exposure and outcome have occurred before
  start of the study
• The study proceeds backwards from effect to cause

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• Select subjects based on their disease status.
• A group of individuals that are disease positive
  (the "case" group) is compared with a group of
  disease negative individuals (the "control"
  group).
• The control group should ideally come from the
  same population that gave rise to the cases.

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• Basic steps in a case-control study
• 1. Selection of cases and controls
• 2. Matching
• 3. Measurement of exposure
• 4. Analysis and interpretation.

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• A 22 table is constructed, displaying exposed
  cases (A), exposed controls (B), unexposed cases
  (C) and unexposed controls (D).
• To measure association is the odds ratio (OR),
  which is the ratio of the odds of exposure in the
  cases (A/C) to the odds of exposure in the
  controls (B/D), i.e.
• OR (AD/BC).

CASE CONTROLS
EXPOSED A B
UNEXPOSED C D
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Case with lung cancer Control without lung cancer
Smokers (less than 5 ) 33 (a) 55 (b)
Non-Smokers (less than 5 ) 2 (c) 27 (d)

The first step is to find out 1. Exposure rates
among cases a/(ac) 33/35 94.2 2.
Exposure rate among the controls b/(bd)
55/82 67
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• If the exposure rate among the cases is more than
  the controls.
• We must see if the exposure rate among the cases
  is significantly more than the controls. This is
  done by using the chi-square test
• It is significant if p is less than 0.05.

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Odds ratio

• It is a measure of strength of association
  between the risk factor and outcome.
• The derivation of the odds ratio is based on
  three assumptions
• ? The disease being investigated is relatively
  rare
• ? The cases must be representative of those with
  the disease
• ? The controls must be representative of those
  without the disease.

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• Odds ratio a.d/b.c
• 33X27/55X2 8.1
• People who smoke less than 5 cigarettes per day
  showed a risk of having lung cancer 8.1 times
  higher as compared to non-smokers.

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• OR is gt 1- "those with the disease are more
  likely to have been exposed,"
• OR close to 1 then the exposure and disease are
  not likely associated.
• OR lt1-exposure is a protective factor in the
  causation of the disease.

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• Case control studies are usually faster and more
  cost effective
• Sensitive to bias (selection bias).
• The main challenge is to identify the appropriate
  control group
• The distribution of exposure among the control
  group should be representative of the
  distribution in the population that gave rise to
  the cases.

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Cohort Study

• It look at cause and proceed to effect
• study before the disease is manifest and proceed
  to study over a period of time for the disease to
  occur.
• Cohort means a group of people sharing a common
  experience.
• Cohort studies are often prospective studies,
  they can be retrospective also, or a combination
  of both prospective and retrospective components
  can be brought in.

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• Steps in a cohort study
• Selection of study subjects
• Obtaining data on exposure
• Selection of comparison groups
• Follow-up
• Analysis.

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CHD Develop CHD does not develop total
Smoker 84 (a) 2916 (b) 3000 (ab)
Non-smoker 87 (C) 4913 (d) 5000 (cd)
Total 171 (ac) 7829 (bd) 8000
The incidence rates of CHD among smokers i.e.
a/(ab) 84/3000 28 per 1000 The
incidence rates of CHD among non-smokers i.e.
c/(cd) 87/5000 17.4 per
1000
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• Then, we must determine if the incidence rate
  among the smokers is significantly more than
  among the non-smokers by using the chi-square
  test.

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Relative risk (RR)

• It is ratio of incidence of the disease among the
  exposed and incidence among the non-exposed.
• RR (incidence of disease among exposed)/
• (Incidence of disease among non-exposed)
• a/(ab)/c/(cd)
• 28/17.4
• 1.6
• If RR is more than 1, then there is a positive
  association
• If RR is equal to 1, then there is no association
• Smokers develop CHD 1.6 times more than
  nonsmokers.

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Attributable risk (AR)

• This is defined as amount or proportion of
  disease incidence that can be attributed to a
  specific exposure.
• It indicates to what extent the disease under
  study can be attributed to the exposure
• (incidence of disease among exposed)
• - (incidence of disease among non exposed)/
• (Incidence of disease among exposed)
• 28-17.4/28
• 10.6/28 0.379 37.9
• 37. 9 of CHD among the smokers was due to
  smoking.

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Differences between case-control andcohort
studies

• Case-control Cohort studies
• Proceeds from effect to ? Proceeds
  from cause to
• cause
  effect
• Starts with the disease ? Starts
  with people exposed
  to risk factor
• Rate of exposure among ? Tests
  frequency of disease
• exposed and those not among
  those exposed and exposed is studied
  those not exposed
• First approach to testing ? Reserved
  for testing
• hypothesis
  precisely define hypothesis

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• Involves small number of ? Involves large
  number of
• subjects
  subjects
• Less time and resources ? More time and
  cost
• 
  intensive
• Suitable for rare diseases ? Difficult to
  conduct for rare
• 
  diseases
• Yields odds ratio ? Yields
  incidence rates,
• 
  RR, AR and population atributable risk
• Cannot yield information ? Information
  about more
• about diseases one other than disease
  is possible
• than selected for

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Randomized Control Studies

• Essential elements are
• Drawing up a strict protocol,
• selecting reference and experimental populations,
  
• randomization,
• intervention,
• follow-up
• assessment of outcome.

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• Randomization is a statistical procedure where
  participants are allocated into groups called
  study and control groups to receive or not to
  receive an experimental therapeutic or preventive
  procedure, intervention.
• Randomization is an attempt to avoid bias and
  allow comparability.

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• Study designs include
• Concurrent parallel
• Crossover type of study designs.
• In the former, study and control groups will be
  studied parallel whereas in the latter all the
  participants will have the benefit of treatment
  after a particular period because the control
  group becomes study group.

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• Types of randomized control studies are
• Clinical trials, e.g. drug trials
• Preventive trials, e.g. trials of vaccines
• Risk factor trials, e.g. trials of risk factors
  of cardiovascular disease, e.g. tobacco use,
  physical activity, diet, etc.
• Cessation experiments, e.g. smoking cessation
  experiments for studying lung cancer.

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What is bias?

• Bias is systematic error that comes in.
• Bias on the part of participants if they know
  they belong to study groupparticipant bias
• bias because of observer if he knows that he is
  dealing with study groupobserver bias
• bias because of investigator investigator bias,
  if he knows he is dealing with study group.
• In order to prevent this, a technique called
  blinding is adopted.

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Concept of blinding

• Single blind trial means participant will not
  know whether he belongs to study group or control
  group.
• In double blind studies, both the participant and
  the observer will not be aware.
• In triple blind study, the participant, observer
  as well as the investigator will not be aware

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• SCREENING
• Active search for apparently healthy people
  is called
• screening.
• It is defined as search for unrecognized
  disease or defect by rapidly applied tests,
  examinations or other procedures in apparently
  healthy individuals.
• Basic purpose of screening is to sort out from a
  large group of apparently healthy
  individualsthose likely to have disease, bring
  those apparently abnormal under medical
  supervision and treatment.
• Efforts to detect cancer before symptoms appear

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• Four main uses of screening are
• (i) case detection,
• (ii) control of spread of infectious diseases
  
• (iii) research purposes especially for
  studying the natural history of chronic diseases
• (iv) screening programs have lot of
  opportunities to educate people.

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• Iceberg phenomenon of a disease explains progress
  of disease from subclinical stages to overt
  manifestation.
• Floating tips represent what the dental surgeon
  see and hidden portion represents unrecognized
  disease. Its detection and control are
  challenges.

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• Iceberg phenomenon of a disease explains progress
  of disease from subclinical stages to overt
  manifestation.
• Floating tips represent what the dental surgeon
  see and hidden portion represents unrecognized
  disease. Its detection and control are
  challenges.

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Q-1

• Incidence of any disease can be known by
• A. Longitudinal study.
• B. Cross-sectional study.
• C. Retrospective study.
• D. Case control study.

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Q-2

• A study in which a particular individual is
  studied at different age is called as
• A. Retrospective study
• B. Prospective study
• C. Cross sectional study
• D. Longitudinal study

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Q-3

• When planning of the entire study is completed
  before data is collected and analysed is
  cosidered as
• A. Retrospective study
• B. Prospective study
• C. Cross sectional study
• D. Longitudinal study

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Q-4

• Case control study is a part of
• A. Descriptive epidemiology
• B. Experimental epidemiology
• C. Analytical epidemiology
• D. Serological epidemiology

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Q-5
  Cases (Oral cancer ) Control (with out oral cancer)
Gutka chewer 20 (a) 15 (b)
Non Gutka chewer 15 (c) 20 (d)
Odds ratio (OR) of getting oral cancer in gutka
chewers is 0.44 1.0 1.7 2.5
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Q-6

• Hypothesis is
• A. A report.
• B. A synopsis.
• C. A theory.
• D. A supposition from an observation.

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Q-7

• Relationship of prevalence (P), incidence (I)
  duration (D) in epidemiology
• IPD
• PID
• DPI
• none

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Q-8

• Constant presence of a disease in a given
  population
• Endemic
• Pandemic
• Epidemic
• Sporadic

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Q-9

• Which one is not the step of descriptive study
• a. Describing the disease.
• 4. Measurement of disease
• 5. Blinding
• 6. Formulate hypothesis

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Q-10

• In descriptive epidemiology, Describing the
  disease in all except
• a. When is the disease occurring
• b. Where is it occurring
• c. Who is getting the disease
• d. How is the disease occure