BASIC EPIDEMIOLOGY
I. EPIDEMIOLOGY
The study of the distribution and the determinants of health related states and events (such as diseases) in specified populations, and the application of this study to the control of health problems.
II. HEALTH CONCEPTS
HEALTH
A complete state of physical, mental and social well-being and not merely the absence of illness (WHO)
DISEASE
Medically defined abnormal changes in the structure or functioning of the human body
ILLNESS
Subjective experience or perception of disease
ie. Impairment, Disability, Handicap, Illness behaviour
EPIDEMIOLOGICAL TRIANGLE
The interaction of HOST, AGENT and ENVIRONMENT factors which produce disease:
HOST
Genetic
Acquired
ENVIRONMENT
Biologic/Physical
Psychosocial
Cultural
AGENT
Biological
Chemical
Physical
III. HEALTH INDICATORS
The health of populations is generally assessed by 3 indicators:
1. Determinants of Health (factors in disease development)
2. Health Status (medical conditions within set populations)
3. Health Consequences (effects on individuals, communities, countries)
Each level incorporates biological, psychological and social elements.
IV. HEALTH STATUS INDICATORS AND DEFINITIONS
Health Status Indicators are Quantitative (Objective) statistical indices which are useful in studying specific diseases or populations
“POPULATION”
A collection of living individuals
“SAMPLE”
A selection of individuals from a Population
Random
Systematic
Stratified
Cluster
“GROUPS”
A subselection of individuals from a Sample
Intervention Group (ie. exposed to risk factor; given treatment)
Control Group (ie. unexposed; given placebo or standard treatment)
“EVENT”
Any specified health outcome.
ie. disease, death
“RATE”
Probability of an event in a defined population during specified time
Expressed as units per population. ie. per 1000.
Eg. Infant mortality rate (the most sensitive measure of a population’s general health)
“RATIO”
Expression of relationship between two events that may or may not be related
Expressed as a ratio. ie. 1:2.
“PROPORTION”
Relationship of a part of an event or group to its whole.
Expressed as a percentage. ie. 25%
“INCIDENCE”
Rate of events in a defined population over time
ie. New cases
A measure of the risk of a disease
“PREVALENCE”
Rate of events in a defined population at a specified time
ie. Total cases
A measure of the extent of a disease
“ATTRIBUTABLE RISK”
Incidence rate of a specified Event among the Intervention group compared to that of the Control group.
ie. A – B.
Eg. Risk due to smoking.
“RELATIVE RISK”
Ratio of the Incidence of a specified Event among the Intervention group to that of the Control group.
ie. A / B.
Eg. Risk in smokers vs non-smokers
“MORBIDITY”
“Pertaining to, affected with or inducing disease”
Formally, measured as Incidence or Prevalence of a specific disease
Informally, used to express to the living consequences of a disease on a person or population
“MORTATILY”
“The quality of being mortal”
Formally, measured as Rate of death in specific populations or from specific diseases
V. INFECTIOUS DISEASES DEFINITIONS
“EPIDEMIC”
Occurrence of a disease within a community in excess of normal expectancy
ie. outbreak
“ENDEMIC”
Habitual presence of disease within a community
“PANDEMIC”
Epidemic which is worldwide
“HOST”
Living object that naturally harbours and nourishes an infectious agent
“CARRIER”
Living object that harbours an infectious agent in the absence of disease
“RESEVOIR”
Living or inert object that harbours an infectious agent in absence of disease
“VECTOR”
Invertebrate animal that may serve as agent of transmission of an infectious agent to vertebrates
“FOMITE”
Inert object that may serve as agent of transmission of an infectious agent
VI. INVESTIGATION OF HEALTH STATUS
QUANTITATIVE STUDIES
Methodologies used to identify and evaluate disease by use of statistical data
2 types of Quantitative Studies:
DESCRIPTIVE STUDIES
Description of disease in terms of person, place and time
Eg. Case Report, Case Series
To generate hypotheses only - cannot draw causal attributions
ANALYTIC STUDIES
Hypothesis tested to determine association between single or multiple factors and an outcome, such as disease
2 types of Analytic Studies:
OBSERVATIONAL STUDIES
Non-controlled settings in which defined populations are studied
CROSS-SECTIONAL/PREVALENCE
1 population; at the present time; Eg. Surveys
To determine Prevalence and identify Risk Factors
RETROSPECTIVE
2 populations compared; in the past; Eg. Case- Control
To determine Odds Ratios
PROSPECTIVE
2 populations compared; in the future; Eg. Cohort
To determine Incidence rates, Relative Risk, Attributable Risk
EXPERIMENTAL STUDIES
Controlled settings in which defined populations are studied by observers
SINGLE-BLIND
Population studied is blinded to its status
Controls for Placebo effect
DOUBLE-BLIND
Population and Observers both blinded to status
Controls for Observer bias
RANDOMIZED
Process of divding the populations
Controls for Confounding
VII. SUMMARY OF QUANTITATIVE STUDIES
QUANTITATIVE STUDIES
ANALYTIC STUDIES DESCRIPTIVE STUDIES
OBSERVATIONAL EXPERIMENTAL
CrossSectional Single Blind
Retrospective Double Blind
Prospective Randomized
HIERARCHY OF EVIDENCE
Randomized Control Trial } Experimental Study
Prospective Study } Observational Study
Retrospective Study } “
Cross-Sectional/Prevalence Study } “
Case Series } Descriptive Study
Case Report } “
Qualitative Study } Qualitative Study
Jeff Unger
BSc, MD, CCFP(EM)
April 2003
REFERENCES
1. Public Health and Preventative Medicine in Canada. Shah, Chandrakant. University of Toronto Press, 1994.