Relative Risk Factor (Hansard, 31 March 1998)

§ Baroness Jay of Paddington

Relative risk provides a measure of the strength of association between a factor and an illness. It is an important way of measuring increases or decreases of risk over time or between different groups by comparing the incidence of an illness or hazard within a population to some baseline (for example if drinkers are twice as likely to suffer from a particular disease as compared with the general population, a factor of 2 may be cited). A stronger association—of greater than 2—is more likely to reflect causation than is a weaker association—of less than gas this is more likely to result from methodological biases or to reflect indirect associations which are not causal. The significance of any such number does though need to be considered in context and from a number of viewpoints.

First, there is a statistical significance: in other words, what confidence is there in the number itself. This will depend on the quality and extent of the available data. Scientists usually express these by giving a confidence interval: rather than by saying that the relative risk factor is 2, they will say that (for example) one can be 95 per cent, certain that it lies between 1.6 and 2.4.

Even when the strength of an association is precisely determined, it is insufficient in itself to confirm a direct causal link between possible cause and an illness. The 32WA strength of association is only one of several criteria which must be considered in the assessment of causation. Other criteria include:

the cause must precede the effect;

the biological plausibility of the association—is the association consistent with other knowledge, e.g., experimental evidence?;

the consistency of the finding—is the same result obtained from different studies using different methodologies elsewhere;

the presence of a "dose-response" relationship—an increased response to the possible cause being associated with an increased risk of developing the illness.

All these factors would be taken into account in trying to pinpoint cause.

The practical significance of risk factors, also needs to be considered and depends on how great is the underlining risk. Doubling a very small probability (risk)—say one in 10,000,000—still results in only a very small risk of illness. Doubling a risk of, say, one in 100 could—depending on its nature—be more serious.

In practice, scientific judgments will be made and debated on a case-by-case basis. The Government can draw on the expertise of independent scientific advisory committees which are constituted to provide balanced judgment on the questions covered above.