Transcript FEM TIPS FØR DU BRUKER MALEN!

Epidemiological darkness
Birger Svihus, professor of nutrition
The experiment - the gold standard of
science
• Randomisation
– Distributes error/other contributing factors
evenly among control groups and intervention
groups
• Intervention
– Gives the scientist control over the factor
studied
• Blinding
– Reduces bias in data collection
Bertrand Russell (1872-1970)
• ”The word ’cause’ is
so inextricably bound
up with misleading
associations as to
make its complete
extrusion from the
scientific vocabulary
desirable.” (Ward,
Medical Health Care and
Philosophy 12, 333, 2009)
John Snow, English physician (18131858)
• Established a link
between water and
cholera by
epidemiological
studies (Freedman,
Statistical Science 3,
243, 1999)
Major observational methods used in
epidemiology
• Correlation studies
– A large number of data are screened to search for
associations between a response (e.g. obesity) and a
factor (e.g. amount of a food)
• Cohort studies
– Healthy persons are grouped according to factors of
interest, and the incidence of a response is registered
over time
• Case-control studies
– Persons with a health problem (cases) are studied in
regards to potential risk factors, and the odds ratio is
compared with a control group without the health
problem
Major problems in observational
epidemiological studies
• Confounding
– Other, unknown factors, can be underlying causes for
both the factor and the response, e.g. the correlation
between icecream consumption and drowning
incidence
• Reverse causation
– A factor that is correlated to a response may not be
the cause for the response, but rather vice versa, e.g.
the correlation between number of firefighters and the
gravity of a fire
• Bias in data collection
Sir Ronald A. Fisher on epidemiology
(Breslow, Journal of the American Statistical Association
91, 433, 1996)
• ”Statistics has gained a place of
modest usefulness in medical
research. It can deserve and retain
this only by complete impartiality,
which is not unattainable by rational
minds … I do not relish the prospect
of this science being now
discredited by a catastrophic and
conspicious howler. For it will be as
clear in retrospect, as it is now in
logic, that the data so far do not
warrant the conclusions based on
them.” (1957, on smoking and lung
cancer)
Schünemann et al., Journal of Epidemiology, Community and
Health 65, 392, 2011)
World Cancer Fund criteria for causation from epidemiological
data (http://www.dietandcancerreport.org/)
Convincing
These criteria are for evidence strong
enough to support a judgement of a
convincing causal relationship.
• Evidence from more than one study type.
• Evidence from at least two independent
cohort studies.
• No substantial unexplained
heterogeneity within or between study
types or in different populations.
• Good quality studies to exclude with
confidence the possibility of
random or systematic error.
• Presence of a plausible biological gradient
in the association.
• Strong and plausible experimental
evidence, either from human studies or
relevant animal models.
Probable
These criteria are for evidence strong
enough to support a judgement of a
probable causal relationship.
• Evidence from at least two
independent cohort studies, or at least
five case-control studies.
• No substantial unexplained
heterogeneity between or within study
types in the presence or absence of an
association, or direction of effect.
• Good quality studies to exclude with
confidence the possibility that the
observed association results from
random or systematic error, including
confounding, measurement error, and
selection bias.
• Evidence for biological plausibility.
Review paper by Rothman and
Greenland (American Journal of Public Health s1,
s144, 2011)
• “What is required is much more than the application of a
list of criteria. Instead, one must apply thorough criticism,
with the goal of obtaining a quantified evaluation of the
total error that afflicts the study. This type of assessment
is not one that can be done easily by someone who
lacks the skills and training of a scientist familiar with the
subject matter and the scientific methods that were
employed. Neither can it be applied readily by judges in
court, nor by scientists who either lack the requisite
knowledge or who do not take the time to penetrate the
work.”
A hierarcical list of criteria to use for
dietary recommendations. The food
should:
• 1. provide enough nutrients
• 2. not provide too much energy and thus cause
obesity
• 3. have a balanced content and quality of
carbohydrates and fat to hinder diabetes 2
and/or atherosclerosis
• 4. not contain too much of ingredients thought to
be carcinogenic, or too little of
ingredients thought to protect against cancer
Review paper by Boffetta (Critical Reviews in
Food Science and Nutrition, 50:13–16, 2010)
• “In conclusion, cancer epidemiology is, to a
large extent, the determination of small effects
and weak associations, and poses major
challenges that are easier to overcome in certain
areas (e.g., genetic epidemiology) than in others
(e.g., environmental or nutritional epidemiology).
Identifying the causal nature of a weak
association is not impossible, but requires large,
well-planned, and well-conducted studies and
supporting evidence from molecular and
experimental studies.”
New dietary recommendations from the
government
• Eat less red meat
The example of red meat
Rich in
essential
nutrients
The example of red meat
Rich in
essential
nutrients
Low in
energy
which
protects
against
obesity
The example of red meat
Rich in
essential
nutrients
Low in
energy
which
protects
against
obesity
Low cho and fat
which protects
against
diabetes/
atherosclerosis
The example of red meat
Rich in
essential
nutrients
Low in
energy
which
protects
against
obesity
Low cho and fat
which protects
against
diabetes/
atherosclerosis
Associated
with colon
cancer
The risk for colorectal cancer due to
red meat (Cross et al., PloS Medicine 4, e325, 2007)
• The risk of developing cancer was 24 % higher
for persons eating 170 gram red meat per day
compared with those eating 30 gram per day
• In Norway, the incidence of colorectal cancer is
around 80 per 100 000. Thus, if the association
is causal, cancer incidence would increase to
100 per 100 000 if meat consumption in Norway
was 30 gram and increased to 170 gram (it is
currently around 80 gram per day)
Review paper on diet and cancer by
Key et al. (The Lancet 360, 861, 2002)
• “Despite extensive research during the last 30
years, few specific dietary determinants of
cancer risk have been established, even for
cancers such as colorectal cancer for which
most researchers agree that diet probably has
important effects.”