یژولویمدیپا رد تّیلع

: ثحب دروم یلصا بلاطم یژولویمدیپا رد ن ا عاونا و طابترا

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یژولویمدیپا رد ن ا عاونا و یتّیلع طابترا

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یژولویمدیپا رد تّیلع یاه لدم عاونا

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تّیلع هرابرد تواضق یاهرایعم

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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 (

تیلع و طابترا

ار ن ا ناو تن هك يا هزادنا هب ) رظن دروم لماع و یرامیب ًلاثم ( ريغتم ود يوق يهارمه : طابترا .

دنت فيب قافتا مه اب ،دور يم راظتنا سناش زا هچن ا زا شيب و داد تبسن سناش هب طقف 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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یفاک و مزلا تلع

Necessary and sufficient cause

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Necessary cause: "A causal factor whose presence is required for the occurrence of the effect”.

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Sufficient cause: A causal factor whose presence is not required for the occurrence of the effect”.

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Any given cause may be necessary, sufficient, both, neither

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تّیلع ات یگتسبمه زا

یگتخاس .1

میقتسم ریغ .2

کی هب کی یتّیلع هطبار یلماع دنچ یتّیلع هطبار » یتّیلع « میقتسم .3

: هطبار عاونا

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) يا هدننك تيوقت رثا ای لقتسم رثا

Independent or synergistic effect

( هیر ناطرس راگیس هیر ناطرس زوتسبز ا هیر ناطرس اوه یگدول ا هیر ناطرس اوه یگدول ا + زوتسبز ا + راگیس 7

یژولویمدیپا رد تّیلع یاه لدم

یژولویمدیپا ثلثم لدم

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خرچ لدم

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تّیلع هکبش لدم

 8

The Epidemiology Triangle

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Host Factors

از یرامیب لماع هب درف خساپ ای و تیساسح ،ههجاوم رب رثؤم یدرف یاه یگژیو

و تادا ع ،یعامتجا یداصتقا تیعضو ،له ات تیعضو ،بهذم ،تیموق ،داژن ،کیتنژ ،سنج ،نس یرامیب هب لاتبا ،هیذغت و کیژولونمیا تیعضو ،ندب کیژولویزیف درکلمع و کیموتان ا راتخاس ،اهراتفر ...

و وراد فرصم ،رگید یاه 10

Environmental factors

ازیرامیب لماع اب نابزیم ههجاوم سناش و ازیرامیب لماع رب رثؤم یطیحم لماوع ادصورس ،تبوطر ،ترارح هجرد : یکیزیف لماوع اه یرامیب یاه لقان : کیژولویب لماوع طیحم یزاسهب ،یتشادهب تامدخ هب یسرتسد ،تیعمج ماحدزا : یعامتجا یداصتقا لماوع

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Agent factors

اه مسیناگرا ورکیم : کیژولویب لماوع یذغم داوم یدایز ای صقن : یا هیذغت لماوع اهوترپ ،امورت : یکیزیف لماوع مومس ،نبرک دیسکونوم ،اهوراد : ی یایمیش لماوع

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Biological Environment The Wheel of Causation Social 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. Temporality 2. Strength of the association 3. Consistency 4. Biological gradient 5. Experiment 6. Plausibility 7. Coherence 8. Specificity 9. Analogy Sir Austin Bradford Hill (1897-1991)

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1.

Temporality

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The causal factor must precede the disease in time.

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This is the only one of Hill's criteria that everyone agrees with.

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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

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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

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Weak associations may be causal but it is harder to rule out bias and confounding.

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3.

Consistency

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The association is observed repeatedly in different persons, places, times, and circumstances.

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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

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A “dose-response” relationship between exposure and disease. Example: Lung cancer death rates rise with the number of cigarettes smoked.

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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

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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.

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Provides strong evidence for causation, but most epidemiologic studies are observational.

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6/7.

Plausibility

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Coherence

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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

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A single exposure should cause a single disease.

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This is a hold-over from the concepts of causation that were developed for infectious diseases. There are many exceptions to this.

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When present, specificity, does provide evidence of causality, but its absence does not preclude causation.

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9.

Analogy

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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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Thank you for your kind attention