Download causation.PH

‫به نام‬
‫خدايي‬
‫كه دراين نزديكي است‬
‫اصول اپيدميولوژي ‪ -‬دانشجويان بهداشت عمومی‬
‫فرزانه مباشري‪ -‬عضو هيأت علمي دانشگاه علوم پزشكي فسا‬
‫ّ‬
‫علیت دراپیدمیولوژی‬
‫مطالب اصلی مورد بحث‪:‬‬
‫•‬
‫•‬
‫•‬
‫•‬
‫ارتباط و انواع آن دراپیدمیولوژی‬
‫ارتباط ّ‬
‫علیتی و انواع آن دراپیدمیولوژی‬
‫انواع مدل های ّ‬
‫علیت دراپیدمیولوژی‬
‫معیارهای قضاوت درباره ّ‬
‫علیت‬
What is a Cause?

Something which has an effect (or Difference).

An event, condition, characteristic (or a combination)
which plays an important role/regular/predicable
change in occurrence of the outcome (e.g. smoking
and lung cancer)
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Characteristics of a cause
1. Must precede the effect
2. Can be either host or environmental factors (e.g.,
characteristics, conditions, actions of individuals,
events, natural, social or economic phenomena)
3. Positive (presence of a causative exposure) or
negative (lack of a preventive exposure)
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)Association & Causation( ‫ارتباط و علیت‬
ً
‫ همراهي قوي دو متغير (مثال بیماری و عامل مورد نظر) به اندازه اي كه‬:‫ارتباط‬
.‫نتوان آن را فقط به شانس نسبت داد‬
Note: Association is not equal to causation.
If the rooster crows at the break of dawn,
then the rooster caused the sun to rise?!!!
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‫علت الزم و کافی‬
Necessary and sufficient cause

Necessary cause: "A causal factor whose presence is
required for the occurrence of the effect”.

Sufficient cause: A causal factor whose presence is not
required for the occurrence of the effect”.

Any given cause may be necessary, sufficient, both,
neither
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‫ازهمبستگی تا ّ‬
‫علیت‬
‫انواع رابطه‪:‬‬
‫‪.1‬‬
‫ساختگی‬
‫‪.2‬‬
‫غيرمستقیم‬
‫‪.3‬‬
‫مستقیم « ّ‬
‫علیتی»‬
‫رابطه ّ‬
‫علیتی یک به یک‬
‫رابطه ّ‬
‫علیتی چند عاملی‬
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‫اثرمستقل یا اثرتقويت كننده اي‬
‫(‪) Independent or synergistic effect‬‬
‫سیگار‬
‫آزبستوز‬
‫آلودگی هوا‬
‫سیگار‪ +‬آزبستوز ‪ +‬آلودگی هوا‬
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‫سرطان ریه‬
‫سرطان ریه‬
‫سرطان ریه‬
‫سرطان ریه‬
‫ّ‬
‫ژ‬
‫مدل های علیت دراپیدمیولو ی‬
‫‪‬مدل مثلث اپیدمیولوژی‬
‫‪‬مدل چرخ‬
‫‪‬مدل شبکه ّ‬
‫علیت‬
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The Epidemiology Triangle
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‫‪Host Factors‬‬
‫ویژگی های فردی مؤثر برمواجهه‪ ،‬حساسیت و یا پاسخ فرد به عامل بیماری زا‬
‫سن‪ ،‬جنس‪ ،‬ژنتیک‪ ،‬نژاد‪ ،‬قومیت‪ ،‬مذهب‪ ،‬وضعیت تأهل‪ ،‬وضعیت اقتصادی‪-‬‬
‫اجتماعی‪ ،‬عادات و رفتارها‪ ،‬ساختارآناتومیک و عملکرد فيزیولوژیک بدن‪ ،‬وضعیت‬
‫ایمنولوژیک و تغذیه‪ ،‬ابتال به بیماری های دیگر‪ ،‬مصرف دارو و ‪...‬‬
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‫‪Environmental factors‬‬
‫عوامل محیطی مؤثر برعامل بیماریزا و شانس مواجهه ميزبان با عامل بیماریزا‬
‫عوامل فيزیکی‪ :‬درجه حرارت‪ ،‬رطوبت‪ ،‬سروصدا‬
‫عوامل بیولوژیک‪ :‬ناقل های بیماری ها‬
‫عوامل اقتصادی‪ -‬اجتماعی‪ :‬ازدحام جمعیت‪ ،‬دسترس ی به خدمات بهداشتی‪ ،‬بهسازی محیط‬
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‫‪Agent factors‬‬
‫عوامل بیولوژیک‪ :‬میکرو ارگانیسم ها‬
‫عوامل تغذیه ای‪ :‬نقص یا زیادی مواد مغذی‬
‫عوامل فيزیکی‪ :‬تروما‪ ،‬پرتوها‬
‫عوامل شیمیایی‪ :‬داروها‪ ،‬مونوکسید کربن‪ ،‬سموم‬
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The Wheel of Causation
Social
Environment
Biological
Environment
Host
(human)
Genetic Core
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Physical
Environment
Web of Causation (Spider web)
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Web of Causation - CHD
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Causal "guidelines" suggested
by Sir AB Hill (1965)
1.
2.
3.
4.
5.
6.
7.
8.
9.
Temporality
Strength of the association
Consistency
Biological gradient
Experiment
Plausibility
Coherence
Specificity
Analogy
Sir Austin Bradford Hill(1897-1991)
1. Temporality

The causal factor must precede the disease in time.

This is the only one of Hill's criteria that everyone
agrees with.

Prospective studies do a good job establishing the
correct temporal relationship between an exposure and
a disease.
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2. Strength of the association

Strong associations are more likely to be causal
because they are unlikely to be due entirely to bias and
confounding.
Example: RR of lung cancer in smokers vs. non-smokers = 9
RR of lung cancer in heavy vs. non-smokers = 20

Weak associations may be causal but it is harder to
rule out bias and confounding.
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3. Consistency

The association is observed repeatedly in different
persons, places, times, and circumstances.

Replicating the association in different samples, with
different study designs, and different investigators gives
evidence of causation.
Note: Sometimes there are good reasons why study
results differ. For example, one study may have looked
at low level exposures while another looked at high
level exposures.
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4. Biological Gradient

A “dose-response” relationship between exposure and
disease.
Example: Lung cancer death rates rise with the number
of cigarettes smoked.

Some exposures might not have a "dose-response"
effect but rather a "threshold effect" below which
these are no adverse outcomes.
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5. Experiment

Investigator-initiated intervention that modifies
the exposure through prevention, treatment, or
removal should result in less disease.
Example: Smoking cessation programs result in lower
lung cancer rates.

Provides strong evidence for causation, but most
epidemiologic studies are observational.
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6/7. Plausibility / Coherence

Biological or social model exists to explain the
association. Association does not conflict with current
knowledge of natural history and biology of disease.
Example: Cigarettes contain many carcinogenic
substances.
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8. Specificity

A single exposure should cause a single disease.

This is a hold-over from the concepts of causation
that were developed for infectious diseases. There are
many exceptions to this.

When present, specificity, does provide
evidence of causality, but its absence does not
preclude causation.
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9. Analogy

Has a similar relationship been observed
with another exposure and/ or disease?
Example: Effects of Alcohol on the fetus
provide analogy for effects of similar
substances on the fetus.
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