third year ebm 2 2013
TRANSCRIPT
Dr Ellie HothersallThis version: September
2013#DundeePublicH
Evidence Based Medicine Part 2:Observational studies
Dr Ellie Hothersall
This version: September 2013#DundeePublicH
Evidence Based Medicine Part 2:
Observational studies
Last week
The world (of evidence based medicine) is very large…
…We are very small
Randomised controlled trials, grouped together into
magnificent meta-analyses are the best way to obtain
evidence that an intervention does or does not have an
effect
Twitter in lectures is interesting but complex (especially if
not everyone can get wifi access)
#DundeePublicH
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Twitter: a second attempt
#Dundeepublich
Switch off wifi on everything except one device – i.e.
phone, OR laptop, OR ipad.
#DundeePublicH
Today…
AIM: To introduce some more key terms in Evidence Based Medicine and Research Methodology
Objectives:
Describe cohort and case control studies and describe the key requirements of both
Define and describe Odds Ratio, Relative Risk, and Absolute Risk
Be able to interpret Odds Ratio, Relative Risk and Absolute Risk
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Learning Outcomes
Describe the purpose of the following types of research
study: Cohort study, Case control study
Understand the types of outcome these studies are likely
to examine and the type of evidence obtained
Describe the key requirements and limitations of these
types of studies
Define, describe and interpret Odds Ratio, Relative Risk, and Absolute Risk
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The Evidence Based Medicine Theme
What? When?
Research 1: The basics Neurology
Research 2: More basics Neurology
Research 3: Understanding p
values
Psychiatry
Research 4: Critical appraisal Psychiatry
Understanding Guidelines Ophthalmology
Understanding Audit Reproduction
A question:
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Why can’t we run a
randomised controlled
trial for everything?
http://ndarc.med.unsw.edu.au/project/randomised-controlled-trial-rct-assess-acceptability-switch-buprenorphine-naloxone-suboxone%C2%AE
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Money
Do you know how much
an RCT costs to run?
http://www.moneysigns.net/british-money-bills/
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We already
do things
http://en.wikipedia.org/wiki/File:Illustration_Salix_alba0.jpg
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http://health.howstuffworks.com/diseases-conditions/rare/10-rare-diseases.htm
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More ethics
http://eofdreams.com/photo/poison/01/
What about interventions
for which we might not get
ethical approval?
Or things out of our
control?
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Unintentional exposure
http://fr.wikipedia.org/wiki/Bombardements_atomiques_d'Hiroshima_et_Nagasaki#DundeePublicH
Radiation exposure
Survivors of Hiroshima and Nagasaki bombs in 1945
Data collected since 1950
Life span study (LSS)
Mabuchi et al. http://www.irpa.net/irpa9/cdrom/VOL.1/V1_19.PDF#DundeePublicH
The structure of a cohort study
http://sph.bu.edu/otlt/MPH-Modules/EP/EP713_CohortStudies/#DundeePublicH
The structure of a cohort study
http://sph.bu.edu/otlt/MPH-Modules/EP/EP713_CohortStudies/#DundeePublicH
The structure of a cohort study
http://sph.bu.edu/otlt/MPH-Modules/EP/EP713_CohortStudies/
http://freeonlinenews.o
rg/weird-diseases/
http://eofdreams.com/death.html
http://joeforamerica.com/2013/02/helping-friends-with-and-without-cancer/
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Types of cohort study
Population
Exposed
Outcome
No outcome
Not exposed
Outcome
No outcome
Prospective
Retrospective
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What do you get from a cohort study?
Estimate of risk in the general population
Ability to pick up infrequent occurrences
Ability to find outcomes you hadn’t
expected
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http://www.teamliquid.net/forum/viewmessage.php?topic_id=98009¤tpage=2892
When is a general
population not a general
population?
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If a condition is
rare, what size
group you need
to detect it?
http://www.theguvnorsassembly.com/forum/the-club/guvnor-for-sale/#DundeePublicH
Rubbish in….?
http://keywordingcentral.blogspot.co.uk/#DundeePublicH
But…
Estimate of risk in the general population
Prospective better for ensuring it really is “general”
Ability to pick up infrequent occurrences
Needs a large (= more expensive) group
Ability to find outcomes you hadn’t expected
Depends on you collecting data (prospective) or data quality (retrospective)#DundeePublicH
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What does a cohort study tell us?
RiskDisease status
Present Absent
Present
Absent
Population
Exposed
Outcome
No outcome
Not exposed
Outcome
No outcome
a
b
c
d
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RiskDisease status
Present Absent
Present a b
Absent c d
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Relative risk (RR)
Pevent when exposed
RR= Pevent when not exposed
a/a+b
= d/c+d
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Relative risk (RR)
Pevent when exposed
RR= Pevent when not exposed
a/a+b
= d/c+d
Also known as a RISK RATIO
Incidence in exposed
Incidence in not exposed
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Interpreting a Relative Risk
Risk Ratio = 1#DundeePublicH
Interpreting a Relative Risk
Risk Ratio <1 Logarithmic scale
Incidence in
exposed <
incidence in
control
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Interpreting a Relative Risk
Risk Ratio >1 Logarithmic scale
Incidence in
exposed >
incidence in
control
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“If relative risk is the guy that drug companies always want to have at the party, absolute risk is the guy who never gets invited, the total buzz kill, the guy who showed up with someone's cousin once in a bad outfit and ended up mumbling to himself in the corner about how everything would be better if people just listened to him.”Heisel, 2010 http://www.reportingonhealth.org/blogs/next-pharma-love-fest-spend-some-time-absolute-risk
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Which is sexier?
50% reduction
in risk
Reduction in risk from
1 in 200,000
to
1 in 400,000
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Which is sexier?
50% reduction
in risk
Reduction in risk from
1 in 200,000
to
1 in 400,000
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www.crestortouchpoints.com
http://realestatebro
wser.files.wordpres
s.com/2012/04/wile-
e-coyote-and-road-
runner-hdforget-the-
fiscal-cliff-cnbc-
jpg_213601.jpg
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P.S.
Calculating Absolute Risk:
Risk in exposed group – Risk in non
exposed group
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Do mobile phones cause
brain cancer?
http://e-healthdiary.com/category/cancer/brain-cancer/#DundeePublicH
How do you answer the question?
Randomised controlled trial
Cohort?
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How a case control study works
http://library.downstate.edu/EBM2/2500.htm#DundeePublicH
Case-control design
Risk factor
No risk
factor
Sample of
CASES(with disease)
Sample of Controls(disease-free)
Risk factor
No risk
factor
Exposure Disease Status
Population
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The direction of time
Cases and controls identified now
Data on past events collected
Backwards in timeCases
Data
Controls
http://www.yianniscove.com/film-of-the-day-back-to-
the-future-1985-dir-robert-zemeckis/
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http://gocampussolutions.com/shop-2/suitcase-storage/
#DundeePublicH www.gocampussolutions.con
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http://www.wexphotographic.com/?/guides/exposure.html#DundeePublicH
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Do mobile phones cause
brain cancer?
http://e-healthdiary.com/category/cancer/brain-cancer/#DundeePublicH
The INTERPHONE study
Cardis et al, 2010. http://ije.oxfordjournals.org/content/39/3/675.full
Regular
use in the
past ≥1
year
Cases Controls
Yes 1262 1174
No 1147 1488
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What does a case control study tell us?
RiskDisease status
Present Absent
Present
Absent
Population
Exposed
Outcome
No outcome
Not exposed
Outcome
No outcome
a
b
c
d
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RiskDisease status
Present Absent
Present a b
Absent c d
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Odds Ratio (OR)
Oddsevent when exposed
OR= Oddsevent when not exposed
a/c
= b/d
axd
= bxc#DundeePublicH
Interpreting an Odds Ratio
Odds Ratio = 1#DundeePublicH
Interpreting an Odds Ratio
Odds Ratio <1 Logarithmic scale
Risk in exposed
< risk in control
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Interpreting an Odds Ratio
Odds Ratio >1 Logarithmic scale
Risk in exposed
> risk in control
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Odds ratios are not odds!#DundeePublicH
Do mobile phones cause
brain cancer?
http://e-healthdiary.com/category/cancer/brain-cancer/#DundeePublicH
The INTERPHONE study
Cardis et al, 2010. http://ije.oxfordjournals.org/content/39/3/675.full
Regular
use in the
past ≥1
year
Cases Controls
Yes 1262 1174
No 1147 1488
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Results
Odds ratio for regular users
compared with not-regular-users
= 0.79
20% LESS likely
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Makes
sense?
Case-Control Studies
Advantages
Appropriate for rare
diseases
Can examine diseases
with a long latency
Can examine many
potential risk factors
Disadvantages
No incidence /
prevalence estimates
Cannot measure disease
onset
Observer bias, Recall
bias
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Pitfalls
Inappropriate controls
Not from same source
Bias in data collection
Dependent on data from the past
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http://bygonebureau.com/2012
/04/13/instructions-for-viewing-
the-bottomless-pit/
http://cisncancer.org/research/how_cancer_is_studied/epidemiological/study_types_03.html
Social
acceptability bias
http://www.mirror.co.uk/news/uk-news/investigation-pubs-underfilling-pints-of-beer-97368
Differences between Cohort
and Case-control studies
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Cohort Case control
Studies exposure to
outcome
Traces outcome back
to exposure
Can measure lots of
outcomes
Can measure lots of
exposures
Starts by identifying
population
Starts by identifying
cases
Relative risk Odds ratio
Prone to bias from loss
to follow upProne to recall bias
Cohort study
Study population
Exposed
Un-exposed
Developed the outcome
Did not develop the outcome
Developed the outcome
Did not develop the outcome
Direction of the research inquiry
Time
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Case-Control study
Study population
With the
Outcome
Without
the outcome
Direction of the research inquiry
Un-exposed
Exposed
Exposed
Un-exposed
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Time
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Research methods:
the Evidence Pyramid
http://www.mstrust.org
.uk/professionals/infor
mation/wayahead/articl
es/16032012_08.jsp
Coming soon…
#DundeePublicHhttp://www.ats.ucla.edu/stat/mult_pkg/faq/general/tail_tests.htm