Epidemiology-causal relationships – Flashcards
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What is a cause?
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an event,condition or characteristic without which the disease would not have occurred
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What are causal factors?
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Exposure/ risk factors- directly influences the occurence of a dz or outcome 1. Agent 2. Environmental 3. Host
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What is an outcome?
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any changes in health stats that can result from exposure to a causal factor
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Definition of effect?
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result of a cause-includes disease or outcome
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Definition of exposure?
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contact with a cause of a particular dz or outcome
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What are agent factors?
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Infection-agent established in host Pathogenic-agent produces dz in host Virulence-measure of severity of dz due to agent
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What are environmental factors?
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temperature and humidity management factors
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What are host factors?
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Natural variation- some have resistance- age,sex,breed exception-> triggering response-comprimise immune system-stress -> populations differ in susceptibility- resistance in populations called HERD IMMUNITY
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How do establish a cause based on evans criteria?
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1. strength of association 2. temporal relationship 3. dose-response relationship 4. biological plausibility 5. consistency 6. elimination 7. reversible associations 8. strength of study designs
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Does association mean causation?
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No
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What are nonstatistical associations?
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Chance NOT A CAUSAL RELATIONSHIP
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What are statistical associations?
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Confidence Intervals- how accurate the association is 95% or 99& usually chosen same population sampled more than once-resulting intervals would bracket true estimate of population approx 95/99% of the time
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Relationship between sample size and confidence?
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Larger sample size-more confidence
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What does positive statistical association indicate?
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causality
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What does negative statistical association indicate?
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protective factor
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What are the 3 types of risk factors?
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1. genetics-predisposition 2. behavioural 3. environmental-infectious agents, sunlight, pollution
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What type of causal factor is a virus?
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necessary and sufficient-one agent to cause dz
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What type of causal factor is respiratory dz in calves?
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necessary but not sufficient- needs 2 or more factors present to cause dz
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What type of causal factor is radiation producing leukemia?
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sufficient not necessary- other factors cause cancer
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What type of causal factor is passive smoking and feline lymphoma?
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not sufficient or necessary- contributes doesnt cause
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Temporal relationship?
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Time it takes/ amount of time between cause and effect
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Dose-response relationship?
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Level of exposure needed e.g. passive smoking
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Biological plausibility?
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does a causal interpretation fit with known facts about mechanisms of dz
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Consistency?
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evidence of a causal relationship has been strengthened where various studies have all come to same conclusions
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Elimination?
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range of diagnostic tests to confirm diagnosis/rule out differentials
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Reversible associations?
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removal of a factor results in an increased or decreased frequency of disease
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Strength of study designs?
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strength of the research designs used to establish a relationship
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List of epi studies from strongest to weakest for providing evidence that an association is causal?
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randomised clinical trials non-randomised studies-> cohort and case studies, cross sectional, case series, case report
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What is used to measure strength of association between causal factor and disease?
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Relative Risk (RR)- cohort studies Odds Ratio (OR)- case-control studies
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What do RR/OR measure?
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Strength of association between causal/exposure factor and dz
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What are factors that impair causal inferences?
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1. long incubation periods-long time b/w cause and effect 2. multiple causes leading to same effect - renal failure 3. pre existing factors required for dz occurence
