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Session 4.1 Basic Concepts of Epidemiology

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

What is Epidemiology?

3 Greek roots:

Epi: Upon. Demos: Human population. Logia: Science. "Science that deals with the distribution and. determinants of diseases in human populations".

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Rationale

Diseases do not occur by chance: there are always determinants for the disease to occur. Diseases are not distributed at random: Distribution is related to risks factors that might need to be studied.

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

The epidemiologic triangle

Agent Disease Host Environment

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

The epidemiologic Triangle

Risk factors

Cancer

Environment

Host

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Risk factors Host Carcinogenesis

Environment

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Risk factors Host Environment

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Risk factors Host Carcinogenesis

Cancer

Environment

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Risk factors Host Carcinogenesis

Cancer 1ry prevention 2ry prevention 3ry prevention

Environment

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Ratio, Proportion, and Rate

A sample with 2 categories, X and Y (e.g. males and females) X : Y = Ratio (Male : Female Ratio) X/(X+Y) % = Proportion A population P with Z events (cases) Z/P per 1,000 (constant) = Rate

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Measures of Risk

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Population

Exposed Unexposed

Risk Factor Others

÷

Relative Risk

Others

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Population

Exposed Unexposed

Risk Factor

Attributable Risk

Others

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Population

Exposed Unexposed

Risk Factor Others

Attributable Fraction %

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

The absolute risk

Synonymous with incidence Means the rate of occurrence of the condition or disease. The basic rate from which relative and attributable risks are derived

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Relative risk

The risk of one group with a factor (e.g. cigarette smokers) compared with the risk of a reference group without such a factor (e.g. Nonsmokers) The ratio of the incidence of the group with the factor to the incidence of the group without the factor. Factors associated with disease by virtue of elevated relative risks are termed risk factors .

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Attributable risk

The amount of the absolute risk (incidence) that can be attributable to one particular factor (e.g. smoking). It is computed by taking the incidence rate of the group with the factor (smokers) and subtracting the rate for the group without the factor (nonsmokers). The excess suffered by the smokers is the attributable risk due to smoking. Indicates the excess of disease due to a factor in that subgroup of the population that is exposed to the factor .

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

The relative risk does not measure the probability that someone with the factor will develop the disease . The individual with the factor might still have a very remote chance of getting the disease, if the disease is rare . Measures the strength of an association between a factor and a certain outcome; A high relative risk points toward causation and is useful in research for the etiology of disease .

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Measurements of risk

Relative Risk = Incidence among exposed / Incidence among unexposed Attributable Risk = Incidence among exposed - Incidence among unexposed Attributable fraction % = Attributable risk / incidence among exposed. By analogy, measure could be calculated for Preventable fraction %.

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Clinical Implications

Although a factor may have a high relative risk, with reference to a common outcome or disease, if that factor is rare in the population, the impact on the population will be small. Patients with familial multiple polyposis have a relative risk of 20 for cancer of the large bowel Incidence of large bowel cancer due to familial multiple polyposis is very small because the attribute is rare it is necessary that a factor both have a high relative risk and be prevalent in the population in order for it to influence the incidence of the disease in the population

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Association and Causation

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Types of Association

An association may be artifactual, noncausal, or causal. An artifactual or spurious association may arise because of bias in the study.. Noncausal associations occur in two ways. The disease may cause the exposure (rather than the exposure causing the disease). The disease and the exposure are both associated with a third (confounding) factor, X, known or unknown. In measuring exposure we are inadvertently measuring the effect of X.

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Causal and noncausal associations

Causal association Exposure Noncausal association (1) Disease (2) ×

Disease

Exposure

Disease Exposure

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Coffee, smoking and CHD

Coffee consumption (cups per day) 0 1-5 6+

CHD mortality in males aged 55-64 (deaths per 1,000 per year) 6 8 12

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Coffee, smoking and CHD

Cigarette consumption (pack. per day) 0 1-2 3+

CHD mortality in males aged 55-64 (deaths per 1,000 per year) 4 10 15

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Coffee, smoking and CHD

Coffee (cups per day) 0 1-5 6+ All

Cigarettes (packs per day) 0 4 6 5 4 1-2 9 10 9 10 30 15 13 16 15

All 6 8 12

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Coffee, smoking and CHD

Cigarette smoking

(confounding factor)

Coffee drinking

CHD mortality

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Coffee, smoking and CHD

Cigarette smoking

CHD mortality

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

5 criteria of causation

consistency of the association. strength of the association. specificity of the association. temporal relationship of the association. coherence of the association.

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

Statistical association present Bias absent Apply 5 criteria satisfied

absent

Conclude the suspected factor is not implicated in etiology Conclude association is artifactual Conclude association is noncausal

present

Not satisfied

Conclude association is causal

By: AS Ibrahim and NNH Mikhail, Ankara Cancer Epidemiology Course, April 2006

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Microsoft PowerPoint - Epi 4.1 Basic Concepts of Epidemiology
Microsoft PowerPoint - Epi 4.1 Basic Concepts of Epidemiology