II MBBS Epidemiology series 1

Association & Causation‘omne ignotum pro magnifico’

Dr. Rizwan S A, M.D.,Assistant Professor,

Department of Community Medicine,VMCH&RI, Madurai.

03.11.2014

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Classification of research methods

Research methods

Observational

Descriptive

Case series, case reports,

CS, cohort

Analytical

Ecological Cross-sectional

Cohort Case control

Experimental

Controlled Uncontrolled

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Procedures in descriptive epidemiology

1. Define the population2. Define and describe the disease3. Measure the disease4. Compare5. Formulate hypothesis

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ExamplesS.No.

Cause Effect

1 Mycobacterium tuberculosis

2 Measles virus

3 Obesity

4 Hypercholesterolemia

5 Excess salt intake

6 Crime

7 Domestic violence

8 Cholera outbreak

9 Polio

10 Congenital anomaly

11 COPD

12 Cancer

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Association

• Terms - association and relationship used interchangeably

• Defined as the co-occurrence of two or more variables at a frequency which is more than that expected by chance

• Association does not mean causation

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Types of association

1. Spurious association2. Indirect association3. Direct (causal) association

I. One to one causal associationII. Multifactorial causation

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Indirect association

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Example of confounder

• Higher maternal age is directly associated with Down's Syndrome in the child

• Higher maternal age is directly associated with Down's Syndrome, regardless of birth order

• Maternal age is directly associated with birth order

• Maternal age is not a consequence of birth order

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Cause

• A cause is something which has an effect• In epidemiology a cause can be considered to

be something that alters the frequency of disease, health status or associated factors in a population

• Necessary and sufficient to produce effect

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Multifactorial causation

Factor 1

Factor 2

Factor 3

Critical change at cellular level Disease

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Multifactorial causation

Factor 1 Factor 3

Critical change at cellular level

Disease

Factor 2

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Hills criteria

1. Temporality2. Strength3. Specificity4. Consistency5. Biological plausibility6. Coherence

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1. Temporality

• Cause always precedes effect• Easy to establish in acute disease, difficult in

chronic disease

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2. Strength

• Relative risk• Dose-response• Duration response• Cessation experiment

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3. Specificity

• Not relevant these days • Due to multifactorial causation theories• ‘not everyone who smokes develops lung

cancer and not everyone who develops cancer smokes.’

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4. Consistency

• Different methods• Different locations• Different disciplines

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5. Biological plausibility

• The mechanism which relates cause to effect with the help of existing knowledge

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6. Coherence

• With already known facts from multiple sources

• Evidences from elsewhere

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Uses of epidemiology

1. To study historically the rise and fall of disease

2. Community diagnosis 3. Planning and evaluation 4. Evaluation of individual's risks and chances 5. Syndrome identification 6. Completing the natural history of disease7. Searching for causes and risk factors

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THANKS FOR LISTENINGEmail your doubts to: sarizwan1986@outlook.com