BRADFORD HILL CRITERIA- GUIDELINES TO EPIDEMIOLOGISTS
In the Year 1965, an Epidemiologist named Austin Bradford Hill developed criterion to assess evidence of causation. These are guidelines that assist an epidemiologist understand the cause for the disease. Epidemiology concentrates on the most probable causes of the disease and not the actual cause. This implies that, epidemiology can be used to understand what the cause of a disease might be but cannot exactly point out the exact cause. This criterion is often considered as a checklist even though it was not meant to be when initially proposed. The Following are the nine points that were proposed by Bradford as criteria to judge the probable cause of a disease.
Strength: A small association does not rule a particular cause out. The larger the association the more likely that the effect is produced as a result of that cause. Statistics are considered when making these decisions because it is imperative that precaution is to be taken so that the disease does not spread.
Consistency: Consistency is not only restricted to one particular individual. The likelihood of an effect increases if the same result is found in different places on different individuals at different times.
Specificity: The more specific an association between a risk factor and an effect, the greater the probability of a causal relationship. If a specific population with a specific disease in a specific area at a specific time has no other rational explanation then causation is extremely likely.
Temporality: Delay between a cause and an effect is also taken into account. The cause should be the reason for the occurrence of the effect, i.e. the effect should occur after the cause. If a particular delay is expected between the occurrence of case and effect, then the effect should happen only after that delay.
Biological Gradient: Scientifically, greater the exposure to a disease causing agent, greater is the risk of getting that disease. In some peculiar cases however, the mere presence of that agent trigger an effect. Other known cases also speak of inverse proportion, i.e. greater the exposure, lesser is the probability of the effect occurring. This is because the body's immune system is now accustomed to that particular agent. Sometimes, the absence of that particular agent after a very long exposure can also lead to a particular effect occurring. As the immune system is accustomed to it, its absence can create certain abnormalities till the body gets rid of the agent. For example, smokers and alcoholics get used to nicotine and alcohol present in their bodies and find it difficult to recuperate if they go cold turkey.
Plausibility: The knowledge of a plausible mechanism between the cause and the effect can be very helpful. Since many diseases are very rare, their mechanism is not widely known.
Coherence: If the laboratory findings complement epidemiological findings, then the probability of that particular cause being responsible for the effect increases. The lack of such findings however, does not rule the cause out.
Experiment: If experimental evidence also compliments epidemiological research, the probability of causal relationship further increases.
Analogy: The effect of similar factors is considered.
A simple mnemonic to remember Bradford Hill criteria is 'ACCESS PTB'