introduction to epidemiology e551a - western...
TRANSCRIPT
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Epidemiology E2200b
Dr. John Koval
Professor of Biostatistics
Department of Epidemiology & Biostatistics
University of Western Ontario
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With thanks to
Dr. Mark Speechley
Professor of Epidemiology
Department of Epidemiology & Biostatistics
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Course Objectives
You will be able to:• understand methodological foundations of applied
human health research• critically appraise original articles about things that are
claimed to be ‘good’ and ‘bad’ for us• perform fundamental calculations using published data• discuss why studies of the same question can get
different answers (and why this doesn’t mean the science is flawed)
• list the bases for criticisms and misunderstandings of the science of epidemiology (know the true rather than the imagined limitations)
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Objectives of Lectures 1&2
You will learn definitions, key concepts, history, modern applications
You will be able to:• correctly use some terminology• describe historical roots, evolution of modern
epidemiology• recognize epidemiology as a basic science
for clinical medicine, public health, health services research, outcomes research, etc.
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Epidemiology: Informal definition
The branch of medical science that helps us identify factors that:
– Keep us healthy (part of ‘health promotion’)– Make us sick (etiologic research)– Help us get better again (therapeutic
research)
“Identifying factors” is NOT the same as “understanding causal mechanisms”
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Some Results
Epidemiological methods have discovered numerous
causal factors of health outcomesThese findings underlie:
massive behavioural change after 1950evidence-based health care & health policy
Disagreements among studies are inevitable and do not signify weaknesses of the methodology.
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Identify factors that:
• Keep us healthy: physical activity, fruits and vegetables in diet, vitamins and minerals, clean air and water, vaccines
• Make us sick: deficiencies of the above factors; smoking; (some) bacteria, viruses, parasites
• Help us get better again: pharmaceuticals, surgery, rehabilitation
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Putative (potential) causal factors precede causal mechanisms
• Often begins with a clinical observation
1774 Dr. Percival Pott – noticed cancer of the scrotum in chimney sweeps, implicated ‘something in soot’
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• Mechanistic knowledge takes years to develop;
we now know that soot contains polycyclic aromatic hydrocarbons that lead to squamous-cell carcinoma
Chimney Sweeps (con'd)
Chimney Sweeps Act :
Sweeps must be at least 8 years old
Sweeps must be provided suitable clothes– and accommodation–
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Other causal risk factors that began with clinical hunches
Exposure• Cigarette smoke
Disease/outcome• Lung cancer (1940s)
Significance? Lung cancer was once very rare. Beginning of epidemic observed among soldiers who had started smoking in WWI. Became the leading cancer death.
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rubella
Exposure
Maternal rubella
(red measles)
Viruses not previously known to cause birth defects (‘teratogenic’).
All women planning pregnancy now immunized
Disease/outcome
Birth defects (1940s)
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What is ‘causation’?Experiment 1:Fred is exposed to A ….. [time passes] …… Fred gets Disease B [turn back the clock, hold everything else constant]
Experiment 2:Fred is not exposed to A … [same time passes] … Fred does not get Disease B
We can define a causal exposure as i) one that is followed by a disease outcome ii) that would not have occurred had the exposure not occurred iii) all else held constant.
√ The perfect research design. √ Proves 100% causal certainty in individuals. (Unfortunately, we cannot reverse time.)
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Dr. Mark’s Magic Potion(A late night infomercial)
• Hi, Friend. Want to ace your grades in university? Well, Dr. Mark has been teaching for years and has concocted a Magic Study Potion in his kitchen laboratory. If it doesn’t increase your marks by one full letter grade, return the unused portion of the product and I’ll cheerfully refund the unspent portion of your money!….And that’s not all!!....Order now and you’ll receive absolutely free….
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Evaluating causal claims
• The Magic Potion claims to causally increase students’ grades.
.
• Is there a way to prove with 100% certainty that any student’s grade was or was not affected by the Magic Potion?
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Causal Certainty Necessity and Sufficiency Criteria
Necessary Cause: The Magic Potion is necessary for increased grades: only students who took my potion increased their marks by a full letter grade; none others did.
Sufficient Cause: The Magic Potion is sufficient for increased grades: every student who took my potion increased their grades.
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A perfect correlation!
Took potion
Increased one letter grade Yes No
Yes Sufficiency(all exposed have outcome)
n/a
No n/a Necessity (no unexposed have outcome)
How many biological, psychological or sociological causes can you name that meet both necessity and sufficiency criteria?
How many can you name that meet even one of these criteria?
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Causation and Correlation
Causation occurs when factor A leads to (or causes) factor B
Correlation happens when factor A and factor B are related, so that when factor A is present, factor B often is present,
and visa versa
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“You can’t prove causation with correlation”
• True, but:– You don't need to know the exact cause before doing
something– We don’t need to understand a causal mechanism to
act to reduce exposure– Dozens of examples exist where epidemiologic
associations have subsequently been demonstrated to be causal.
– If an association is causal, every day we fail to act out of scientific prejudice, people will needlessly get ill or even die.
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We will be wrong sometimes.
Example: in an investigation of an outbreak of Hepatitis A
hot dog sausages were implicated.
The whole consignment of sausages was thrown out.
People were spared from the disease, although the actual mechanism was not clear.
It turns out that the source of the bacteria was actually the relish.
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“You can’t prove causation with correlation” is true, but…
• You can’t prove causation without correlation either.
• All identified causes began with observed correlations.
The problem isn’t correlation, it’s failure to control for CONFOUNDING – other explanations that could account for the correlation.
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How to prove causation?ApproachesBest: expose Person A,
observe; go back in time, remove exposure, observe and compare
2nd Best: random assignment to exposure (Experimental)
3rd Best: observe people in different exposure groups
(Observational)
LimitationsCan’t do time travel:“counterfactual”
Unethical with negative outcomes
Often impractical (time)
Potential for confounding*
*Latin, confundere (pour together; confuse)
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2nd best:The RCT (Random Controlled Trial)• Randomize students to Magic or Placebo Potion:
All known and unknown factors are distributed by chance
• Collect data on factors that could affect grades, compare two groups at baseline, should be similar as the sample size increases
• If imbalanced, can statistically adjust final estimates• Observe between-group difference in grades
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3rd best: Observational Designs• Are not true experiments• People select themselves into exposures • Unknown or unmeasured factors
(confounders) could be the true cause of any observed difference
• As our theory improves (as we can explain a larger portion of the variation in outcomes) so does our ability to estimate the true causal effect of any single factor
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The role of confounding
Cigarette Smoking
DiseaseCoffee consumption
Non-causalassociation:heavy smokerstend to be heavy coffeedrinkers
True causal effect
Spurious association
Smoking, a true cause of disease, will confound (bias) the association between coffee and disease. The apparent association
with coffee is due to the correlation between coffee and smoking.
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Confounding (con'd)If you measure association of smoking and cancer
in the presence of a measurement of coffee consumption, the true effect of smoking will be diminished
Coffee consumption is a confounder of the Smoking – lung cancer relationship
Determination of actual risk factor and actual confounder depends on other (clinical) studies
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Malaria (‘bad air’): A classic case of confounded association
Swamps (musty air) Malaria
Highlands (fresh air)No
Malaria
Confounder____________
True cause
Spurious association
Solution; leave swanp, What’s the true cause (vector) of malaria?
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Epidemiology* (definition)
• “the study of the occurrence and distribution of health-related states or events in specified populations, including the study of determinants influencing such states, and the application of this study to control health problems” (Porta M. A Dictionary of Epidemiology, 5th ed, 2008:81). (emphases added)
*From Greek; epi (upon) dēmos (people), logos (word, reason)
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“distribution” (Porta, 2008)“The complete summary of the frequencies of the
values or categories of a measurement made on a group of persons. The distribution tells either how many or what proportion of the group was found to have each value (or each range of values) out of all the possible values that the quantitative measure can have”.
Usually presented broken down by characteristics such as person, place, and time.
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Age distribution of percentage of pregnancies ending in miscarriage/stillbirth, by age of women at
end of pregnancy, Canada, 1974 and 1992
0
5
10
15
20
25
Allages
15-19 20-24 25-29 30-34 35-39 40-44
1974
1992
Source: Health Reports, Summer 1996, 8:13
%
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“determinants”
“any factor that brings about change in a health condition or other defined characteristic. (Porta, 2008).
Identifying possible (and probable) causal factors is not the same as explaining causal mechanisms
If a factor is causal, reducing exposure will reduce outcome even if we don’t understand the mechanism
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“study” can be:
• Surveillance – (e.g. mandatory disease reporting)
• Descriptive (hypothesis generating)– (e.g. proportion of pregnancies that end in
miscarriage/stillbirth, by characteristics of person, place and time)
• Analytic (hypothesis testing)– (estimates of X-Y association from
observational studies)
• Experiments (clinical trials)
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Analytic Epidemiology: Primary role is etiologic*
Exposures ('determinants')
For example,– Physical (ionizing
radiation)– Chemical (lead)– Biological (needlesticks)
– Social: educational attainment, poverty
– Behaviours: tobacco, diet
Outcomes (‘health related states and events’)
For example,• Diseases with biological
models
• Illnesses without biological models
• Injuries• Birth outcomes• Psychological states such as
QOL (Quality of Life)*Greek, aitia (cause)
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Key concept: Reliable case definition
• Case definition: A set of criteria (not necessarily diagnostic criteria) that must be fulfilled in order to identify a person as a case of a particular disease (Porta, 2008:32)
– Clinical or Laboratory criteria or both– Scoring systems with points that match disease
features (e.g. Multiple sclerosis)
• Reliability: The degree to which the results obtained by a measurement or procedure can be replicated (Porta, 2008:214)
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Key concept: Risk
RISK(def): The probability that an event will occur, e.g., that an individual will become ill or die within a stated period of time or age. (Porta, 2008:217)
Major aim of Epidemiology is to quantify the risk of developing disease or other negative health state posed by various exposures (molecules, microorganisms, environments, behaviors).
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Probability
• Causation of health and illness is extremely complex
• Even widely agreed upon causes fail to meet necessity and sufficiency:– “Grandma smoked a pack a day and died peacefully
in her sleep at 110, and Uncle Elmo got lung cancer and never smoked”.
• We need to rely on probability statements: the probability of an outcome is 2, 3, 4.. times higher among exposed than unexposed
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Observed versus predicted probability
Average (predicted) risks estimated from groups, used to advise individual patients: (e.g. risk of adverse surgical outcome; risk of cancer recurrence)
But! individuals will either have (risk = 100%) or not have (risk = 0%) an outcome over a specified time period (you can’t have ‘32% of a stoke’).
– Estelle, 28, never-smoker, former Varsity volleyball player, has a stroke. Observed individual risk of stroke for that year = 100%
– Jerome, 75, high blood pressure, smoker, does not have a stroke. His observed individual risk for that year = 0%
People like Estelle face a very low predicted risk; people like Jerome face a much higher predicted risk
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History of Epidemiology
Epidemiology is a young science with ancient roots in the study of epidemics (def: “The occurrence in a community or region of cases of an illness, specific health related behavior, or other health-related events, clearly in excess of
normal expectancy.” Porta, 2008:79) “Clearly in excess” differs by disease and time
frame (e.g. H1N1 or Lung Cancer)
Began with communicable diseases; methods have been adapted for chronic diseases and other health states and events (injuries, birth outcomes, etc)
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Demons, Miasms* and Germs
Epidemiologic insights (e.g. events are not random) are clear in the writings of Hippocrates 2500 years ago.
Millenia passed before we had the intellectual foundation to scientifically test 2 competing hypotheses about the causes of epidemic diseases
Key period: 1850s England: Drs. John Snow (cholera) and William Budd (typhoid fever)
*From Greek, miainein (to pollute).
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2 theories of epidemic disease
Miasmatic (miasma)• Air has a ‘bad quality’• Rotting organic matter• ‘Miasma’ could be
passed from cases to susceptibles in contagious diseases
Contagion• Invisible entities• Spread through direct
contact, droplet spread or contaminated fomites
Most physicians supported miasma; it explained the facts better:• didn’t know that asymptomatic people could be infectious
(‘well carriers’)• Didn’t know about immunity
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1850s England: Urbanization, industrialization, poverty, crowding,
filth and epidemic disease
Increasingly scientific medical profession continued to favour miasmatic theory over contagion:
• London, 1854: Cholera epidemicwas it miasma or germs?
• How to prevent/stop epidemic
Lack of Sanitation
“Miasms”
GermsDisease
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William Farr Deaths from Cholera in 10,000 Inhabitants by Elevation of
Residence above Sea Level, London, 1848-1849Elevation above Sea Level (ft) Number of Deaths
<20 120
20-40 65
40-60 34
60-80 27
80-100 22
100-120 17
340-360 8
Data from Farr W: Vital Statistics: A Memorial Volume of Selections from the Reports and Writings of William Farr (edited for the Sanitary Institute of Great Britain by Noel A. Humphreys). London, The Sanitary Institute, 1885.
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John Snow, M.D. (1813-1858)
www.ph.ucla.edu/epi/snow.html
• 1847- theory that cholera is communicable and waterborne
• Used spot maps of cases’ residences, compared to location of public water pumps
> 500 cholera fatalities within 250 yards of Cambridge and Broad Streets in a 10 day period.
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Snow's diagram
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Snow (con'd)
Eventually convinced Parish authorities to remove Broad Street pump handle during August-September 1854 epidemic
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Modern day view
The pump on
Broadwick street
Plus the Pump Pub
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Cases of Cholera by date of onset, London, Aug. 19 – Sept. 19, 1854
0
20
40
60
80
100
120
140
160
Fatal attacksDeaths
Epidemic curve adapted from Roht et al, 1982:300
Pump handleremoved
August September
f
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“Natural Experiment”London England, ~1853
• 2 major water suppliers: Lambeth, and Southwark & Vauxhall
• Lambeth moved their intake to a cleaner section up river
• Interviewed household members to ascertain which of two companies supplied their water
• Compared 1853 cholera cases according to water company (retrospective study)
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Cholera mortality by water supply, 1st seven weeks of epidemic
(Roht et al, 1982:304)
Water Supply
# houses Cholera Deaths
Deaths/10,000 houses(risk)
Southwark & Vauxhall
40,046 1,263 315.4
Lambeth Co.
26,107 98 37.5
Rest of London
256,423 1,522 59.4
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Epidemiologic measures of association: Relative Risk*
One form of Relative Risk = Risk Ratio
Deaths/10,000 exposed (S&V) = 315.4 = 8.4Deaths/10,000 unexposed (Lambeth) 37.5
Mortality was 8.4 times more common in S&V houses than in Lambeth houses.
Based on these non-experimental (non-randomized) findings, who here would choose S&V?
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The Establishment reacts:
• The Lancet: “not by any means conclusive”• Royal College of Physicians: “theory as a whole is
untenable” … continued to support “foul or damp air” as the cause
• Board of Health Medical Inspectors: “We see no reason to adopt this belief” (1854)
• “far-fetched doctrine”(Chapman, 1866)• 1884, Robert Koch (Nobel, 1905) identified Vibrio
Cholerae, made no mention of Snow’s work
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This is, unfortunately, not uniqueMany epidemiologic findings, even after multiple
replications and systematic testing and rejection of bias explanations, are stubbornly resisted. Why?
• economic self-interest • resistance to behavioral change • unwillingness to admit past practices killed people
Unfortunately, isolated first findings are often given the most sensationalistic media coverage