Causality..Epidemiology

Philosophical background

Causation

Deals with the relationship between cause and effect

The Classical period

Aristotle-doctrine of four causes

Material cause

• What 'stuff' (matter) a thing comprises-some combination of the four elements:
• Earth
• Air
• Fire
• Water

Formal cause

• The 'form and pattern' of a thing, or those properties without which a thing would not exist as it does

Efficient cause

• Which is the maker of a thing (and by which the formal cause is therefore explained)

Final cause

• Which is the purpose of the thing (and therefore, in the case of natural things, usually synonymous with the formal cause)

The Scholastics

• Christian mediaeval philosophers (termed the 'Scholastics') generally endorsed Aristotle's ideas, but focused on God as the efficient cause of all things
• Individuals were secondary efficient causes of things
• God as the primary cause was a subject of debate during this period.

The 'Modern' period

Explain events mathematically in terms of

• ‘How' (description)
• ‘Why' (explanation)

Causal inference

Scientific conclusions are derived by two methods of reasoning:

• Deduction
• Induction

Deduction

• Deduction is arguing from the general to the particular; that is, a general case is established, from which all dependent events are argued to be true
• Example: Thus, if one posits the truth of the general proposition 'all dogs are mammals', it follows by deduction that any particular example of a dog will be a mammal

Induction

• Arguing from the particular to the general
• Example: A dog may be vaccinated against distemper virus, and shown to be immune to challenge with the agent, from which the conclusion is drawn that the vaccine prevents distemper in all dogs.

Why to identify cause?

• Epidemiological studies are undertaken to identify causes of disease so that preventive measures can be developed and implemented, and their subsequent effectiveness identified
• Investigations of cause are usually based on inductive reasoning

Methods of acceptance of hypotheses

Accept (or reject) a causal hypothesis by four methods

• Tenacity-disregards opinion of others
• Authority
• Intuition
• Scientific inquiry

Koch's postulates

Koch in the late 19th century formulated postulates about cause of infectious disease

These postulates state that an organism is causal if:

• It is present in all cases of the disease
• It does not occur in another disease as a fortuitous and non-pathogenic parasite
• It is isolated in pure culture from an animal, is repeatedly passaged, and induces the same disease in other animals.

Koch's postulates brought a necessary degree of order and discipline to the study of infectious disease

Evans' rules

Alfred Evans (1976) has produced a set of rules that are consistent with modern concepts of causality:

• The proportion of individuals with the disease should be significantly higher in those exposed to the supposed cause than in those who are not
• Exposure to the supposed cause should be present more commonly in those exposed to the supposed cause than those without the disease, when all other risk factors are held constant
• The number of new cases of disease should be significantly higher in those exposed to the supposed cause than in those not so exposed
• Temporally, the disease should follow exposure to the supposed cause with a distribution of incubation periods on a bell-shaped curve
• A spectrum of host responses, from mild to severe, should follow exposure to the supposed cause along a logical biological gradient
• A measurable host response (e.g., antibody, cancer cells) should appear regularly following exposure to the supposed cause in those lacking this response before exposure, or should increase in magnitude if present before exposure; this pattern should not occur in individuals not so exposed
• Experimental reproduction of the disease should occur with greater frequency in animals or man appropriately exposed to the supposed cause than in those not so exposed
• Elimination (e.g., removal of a specific infectious agent) or modification (e.g., alteration of a deficient diet) of the supposed cause should decrease the frequency of occurrence of the disease
• Prevention or modification of the host's response (e.g., by immunization or use of specific lymphocyte transfer factor in cancer) should decrease or eliminate the disease that normally occurs on exposure to the supposed cause

Variables

A variable is any observable event that can vary:

Examples:

• Weight
• Age of an animal 
• Number of cases of disease

All relationships and associations should be biologically and epidemiologically credible