Veterinary Epidemiology and Zoonosis

METHODS OF HYPOTHESIS

Four major methods of arriving at a hypothesis

• Method of difference
• Method of agreement
• Method of concomitant variation
• Method of analogy

Method of difference

• If the frequency of a disease is different in two different circumstances, and a factor is present in one circumstance but is absent from the other, then the factor may be suspected of being causal.
• For instance, BSE occurred more frequently in places where meat and bone meal is mostly used in feed than in places where it is not. A defect of a hypothesis based on the method of difference is that several different factors may be considered as possible causes.
• The value of a hypothesis generated by this method is reduced if many alternative hypotheses can be formulated.

Method of agreement

• If a factor is common to a number of different circumstances in which a disease is present then the factor may be the cause of the disease. Thus, if a batch of meat and bone meal was associated with salmonellosis on widely different types of pig farms, and this was the only circumstance in common, then the causal hypothesis, that the disease was caused by contamination of that batch ­is strengthened.

Method of concomitant variation

• This method involves a search for a factor, the frequency or strength of which varies continuously with the frequency of the disease in different situations. Thus, the distance over which cattle are transported before slaughter appears to be related to the occurrence of bruises in their carcasses.

Method of analogy

• This method of reasoning involves comparison of the pattern of the disease under study with that of a disease that is already understood, because the cause of a disease that is understood may also be the cause of another poorly understood disease with a similar pattern.
  • Example: Some mammary tumours of mice are known to be caused by a virus, therefore some mammary tumours of dogs may have a viral cause. Evidence by analogy is not evidence of fact.
• It can point to probabilities and confirm conclusions that may be reached by other means but can be dangerously misleading.
• When attempting to establish a causal association, five principles should be considered
  • The time sequence of the events
  • The strength of the association
  • Biological gradient
  • Consistency
  • Compatibility with existing knowledge

Time sequence

• Cause must precede effect.

Strength of association

• If a factor is causal, then there will be a strong positive statistical association between the factor and the disease.

Biological gradient

• If a dose-response relationship can be found between a factor and a disease, the plausibility of a factor being causal is increased. This is the basis of reasoning method of concomitant variation.

Consistency

• If an association exists in a number of different circumstances, then a causal relationship is probable. This is the basis of reasoning by the method of agreement.

Compatibility with existing knowledge

• It is more reasonable to infer that a factor causes a disease if a plausible biological mechanism has been identified than if such a mechanism is not known. Thus, smoking suggested as a likely cause of lung cancer because chemical and environmental pollutants are known to have carcinogenic effect on laboratory animals.