Criteria for Causal Association

•Name: Kaushik.P•Roll Number: 4050•Guide: Dr.Akila.G.V

Contents

•Introduction•Types of causal association•Criteria for causal association•Summary•References

Introduction

Association may be defined as the concurrence of two variables more often than would be expected by chance. Epidemiological studies determine various associations between an exposure and a disease.Further, its important to find out whether the exposure is causal for the disease or not.

Types of association

•Spurious association•Indirect association•Direct (causal) association

a) One-to-one causal associationb) Multifactorial association

Spurious association Association between a disease and suspected factor may not be real.

Indirect association Associations which at first appeared to be causal have been found on further study to be due to indirect association. This is due to a confounding variable.

Example for Spurious association:

PMR of 5.4 PMR of 27.8

Example for Indirect association:

High altitude Goiter

Direct (Causal) associationa)One-to-One causal association Two variables(AB) are stated to be causally related if a change in A is followed by a change in B.b)Multifactorial association Considered when the etiology is multifactorial. All the causal factors can act individually or cumulatively to produce the outcome.

One to one causal association-

Measles virus Measles

Multifactorial association-

CHD

Criteria for Causal Association

Bradford Hill’s criteria for making causal inferences-1.Strength of association2.Dose-Response relationship3.Lack of temporal ambiguity4.Consistency of findings5.Biologic plausibility6.Coherence of evidence7.Specificity of association

1.Strength of association

• Measured by the relative risk (or odds ratio).

• The stronger the association, the more likely it is that the relation is causal.

• Relative risk is the ratio of the incidence of the disease among exposed and the incidence among non-exposed.

Example-Risk for development lung cancer

8.6 times higher in smokers than in non-smokers.

2.Dose-Response relationship

• As the dose of exposure increases, the risk of disease also increases

• If present, it is strong evidence for a causal relationship.

• Absence of a dose-response relationship does not necessarily rule out a causal relationship.

• In some cases in which a threshold may exist, no disease may develop up to a certain level.

Daily average cigarettes smoked

Relative risk of developing lung cancer

1 - 14 6.7

15-24 12.3

25+ 23.7

Example-

3.Lack of temporal ambiguity

• Exposure to the factor must have occurred before the disease developed

• The temporal relationship is important in regard to the length of the interval between exposure and disease

• It’s easier to establish a temporal relationship in a prospective cohort study than in a case-control study or a retrospective cohort study.

Example- Consumption of contaminated food should precede the symptoms of food poisoning.

4.Consistency of findings

• The relationship should be found consistently in different studies and in different populations.

• Unless there is a clear reason to expect different results, replication of the findings should be there.

5.Biologic plausibility

• Biologic plausibility refers to coherence with the current body of biologic knowledge

• Epidemiologic findings should be consistent with existing biologic knowledge.

• Example- Carcinogens from cigarette smoke deposits in the lung over a period of time leading to lung cancer.

6.Coherence of evidence

• If a relationship is causal, we would expect the findings to be consistent with other data.

• For the appraisal of causal significance of an association it should be coherent with known facts that are thought to be relevant.

Example- Peptic ulcer disease

• Prevalence of H.pylori is same in men as in women. Incidence of duodenal ulcer in both have been proved to be equal in recent years.

• Prevalence of peptic ulcer disease is believed to have peaked in the latter part of 19th century cause of poor living standards.

7.Specificity of association

• Association is specific when a certain exposure is associated with only one disease

• When specificity of an association is found, it provides additional support for a causal inference

• Absence of specificity in no way negates a causal relationship.

Example- Prevalence of H.pylori in patients with duodenal ulcer is 90% to 100%.However, it is found even in some patients of gastric ulcer and even in asymptomatic individuals.

Few other criteria which might be useful are:• Cessation of exposure-   Risk of the disease declines when exposure to the factor is reduced or eliminated.

• Consideration of alternate explanations- Extent to which the investigators have taken other possible explanations into account and the extent to which they have ruled out such explanations are important considerations.

Summary:

• Association in epidemiological studies and its types

• Causal association• Bradford Hill’s criteria for

causal association

References:

• Gordis L. Epidemiology. 4th ed. Saunders Elsevier : Philadelphia ; 2009. Pg 227 to 246.

• Park K.Textbook of Preventive and Social Medicine. 21th ed. Bhanarasidas Bhanot : Jabalpur (India); 2011. Pg 84 to 87.