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Summary Epidemiology (AM_1179)
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Epidemiology, Daniëlle Band, 2702582Lecture 1. Study designs
Epidemiology comes from three Latin words: (1) Epi means upon, (2) demos means people, and (3) logos
means study.
Epidemiology is the study of the distribution and determinants of health-related states or events in
specified populations and application of the study to the control of health problems. Distribution
consists of the frequency and the pattern. Frequency refers not only to the number of health events in
a population, but also to the relationship of that number to the size of the population. Pattern refers to
the occurrence of health-related events by time, plica and person. Determinants causes, and other
factors that influence, the occurrence of disease and other health related events. Health related states
are not only diseases but also injuries and disabilities. Application is the practice implications and
recommendations.
Practice questions
o Which one of the following does this statement relate to: Compare food histories
between persons with Staphylococcus food poisoning and those without – Determinants
o Which one of the following does this statement relate to: Compare frequency of lung
cancer among smokers with frequency in general population – Determinants
o Mark on a map the residences of all children born with birth defects within 2km of
hazardous site – Distribution
o Graph the number of cases of congenital syphilis by year for the country – Distribution
o Recommend that close contacts of a child recently reported with meningococcal
meningitis receive Rifampin – Application
John Snow (physician) is the father of field epidemiology. In 1850s in London there was
a massive cholera outbreak. Nobody knew what cholera was or what caused it. Many
thought that this was caused by bad air (Miasma theory). That is when John snow saw
many patients with cholera and he looked into what was happening. He looked at a
street (broad street) and he saw that many household received water from the same
hand pump. Many patients get cholera also used water from this pump. John Snow
went to the pump and took out the handle from the pump causing a change in the
source of the water. This resulted in less people getting sick and
getting cholera. This was back then seen as revolutionary. John snow
did not stop there and went further. He looked into the map of
London and looked where everyone got water from. There were three different water companies. This
showed that Southwark and Vauxhall company was the most problematic. Most companies got water
from the Thames. Southwark and Vauxhall got water downstream from the sewer which causes more
contamination in the water.
Epidemiology researches look at the exposure resulting in a specific
outcome with association and causal relationship in between. What is the
association between different variables and risk factors? Epidemiology can
be used in different fields: (1) public health surveillance, (2) field
investigations, (3) analytic studies, (4) evaluation public health services, (5)
and policy development.
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,Epidemiology, Daniëlle Band, 2702582
Study designs
Observational and experimental studies
There are two types of epidemiological studies. (1)
observational studies (consisting of analytical and
descriptive studies), which observe and measure a
set of characteristics in a defined sample
population. (2) Experimental studies, which
manipulates a characteristic within a defined
sample population with the purpose of studying the
effect this has.
An experimental study is a Randomized control trial (RCT) which sets the gold standard of clinical
research. It resembles the cohort study in several respects, with the important exception of
randomization of participants to exposures. If properly designed and done, a randomized controlled
trial is likely to be free of bias. This study is always prospective (future orientated) in design and it has
two key features, (1) it has a control arm and (2) there is randomization. The basic design has two arms,
intervention and control arm (e.g. drugs vs placebo, or physical therapy vs surgery), but there are more
arms possible (e.g. old drug vs new drug vs placebo). Randomization is a blinded allocation to one of the
intervention groups based on chance. Each person has an equal chance of being assigned to each
treatment, but the treatment to be given cannot be predicted. This is done to make sure there is no
selection bias or confounding, and to get rid of the effect of “indication for intervention”, which means
that the prognosis influences (unconscious) allocation to treatment by doctors. There are two times of
blinded allocation, (1) signal blinded (patients do not know what they get, but the researchers do), and
(2) double blinded (both the patients and the researcher does not know what the patient gets, but
another research group does know).
Advantage RCT Disadvantage RCT
o The gold standard of clinical research o Expensive
o When properly implemented, random allocation precludes o Cannot be used for
selection bias. harmful substances
o This study design is that it eliminates confounding bias, both
known and unknown.
o High internal validity (it measures what it sets out to measure)
Observational studies (Descriptive epidemiology and analytical epidemiology)
Observational studies consist of two Five basic W questions (Descriptive studies cover who,
types. (1) Descriptive epidemiology when and where. Analytical studies cover why.)
studies the pattern of diseases and • What is the disease or condition being studied?
focusses on the questions who? Where? • Who has the disease in question? (= person)
When? (e.g. what is the frequency of • When is the disease common or rare? (= time)
symptoms of “anxiety” among students?). • Where does the disease arise? (= place)
(2) Analytical epidemiology studies the • Why did the disease or condition arise?
determinants of the disease and focusses on the question why? It is all about comparing persons,
groups, etc (e.g. Are epidemiology students more likely to have symptoms of “anxiety” than theology
students?)
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, Epidemiology, Daniëlle Band, 2702582
Descriptive
Descriptive studies have several important roles in medical research. They are often in the first foray
into a new disease or area of inquiry, in the first scientific “toe in the water”. There are two different
types of descriptive studies. (1) Individual, consisting of case report, case series report, and cross
sectional (prevalence) studies. A case report is a novel occurrence is described in a detailed report. A
case series report is a group of case reports involving patients who were given a similar treatment or
obtained the same symptoms of a disease. A cross-sectional study analyzes data of variables collected
at one given point in time across a sample population. (2) Aggregate, consisting of ecological studies.
Ecological studies are correlational studies that look for associations between exposure and outcomes
in populations rather than individuals. It looks into aggregate and not individual data and thus the unit
of observation is not a person, but an entire group or population. An important limitation of ecological
studies is ecological fallacy which is the inability to link the exposure to the outcome in individuals and
the inability to control for confounding. The data at country level cannot be used to tell stories about
individuals.
Analytical
Analytical studies have a
very important role in
medical research, for
example in understanding
the risk factors. There are
three types of analytical studies.
(1) In cohort studies, study groups are defined by exposure (exposed
vs non-exposed). These groups are followed for a defined period to
assess the proportion that develop an outcome/disease of
interest. A cohort is a group of persons that share at least one
characteristic. These cohort studies can be both fixed (once you
are in, you cannot leave the cohort and it is determined by a
certain event or occurrence.) or dynamic (migration occurs, i.e.
members drop out and new members join, and it is determined
by a certain state or situation). There are different types of
cohort studies. (a) Concurrent/prospective in which a group of subjects
are followed over a period of time to gather information and record the
development of outcomes. (b) Retrospective in which researches look
backwards and examines exposures of suspected risk or protection
factors in relation to an outcome that is established at the start of the
study. (c) Ambidirectional in which the study is both retrospective and
prospective.
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