There is so much ignorant ballix spoke about "scientific proof" that I paste here (thanks to ISIS) the Bradford Hill criterial needed to build a link between a problem and an environmental agent.
(1) Strength of the association. The death-rate from lung cancer was over nine times as high in smokers as in non-smokers; in heavy smokers, it was more than twice that again. This was obviously much stronger supporting evidence than if the rates had only been slightly higher.
(2) Consistency: Are we talking about the result of a single study, or of several, and if there is more than one, were they all done in the same way or were they really different? Bradford Hill pointed out that according to a committee advising the US Surgeon General, 36 different inquiries, not all using the same methodology, had found an association between smoking and lung cancer. That does not rule out the possibility that the same fallacy was at work in all of them, but it strengthens the case.
(3) Specificity: If a disease occurs only in one group of people and if there are no other diseases that occur only in this group, this is strong evidence for cause and effect. In fact, while the death rates for smokers are higher for many causes of death, the increase is much greater for lung cancer than for the others, so this criterion is still satisfied.
(4) Temporality: While cause obviously has to come before effect, it is not always obvious which of two events was really first. If people who smoke are more likely to die from lung cancer, does that mean that smoking causes cancer or is it that the sort of people who are predisposed to lung cancer are also likely to adopt a life style that includes smoking? Here the obvious explanation is correct – smoking does cause lung cancer – but it is a question we should ask.
(5) Dose response: Does increasing the purported cause increase the effect? In the case of smoking and lung cancer, the increase in death rate rises linearly with the number of cigarettes smoked per day, and this is strong supporting evidence. On the other hand, in many cases there are threshold or trigger effects, and then there will be no dose response. Drinking two glasses of poison doesn’t make you twice as dead.
(6) Plausibility: Is the cause-effect relationship plausible? Ideally, we would like to be able to find the mechanism that links cause and effect, but often this is not possible; if it were there would be no problem. We can, however, ask if it is at least plausible that A could be the cause of B. Hill immediately warns, however, that what is considered plausible changes in time. In the nineteenth century, for example, it was thought totally implausible that doctors not washing their hands could be responsible for the deaths of women in maternity wards.
(7) Coherence: Does the claim that A causes B seriously conflict with what we know about B? This is really a companion to the plausibility criterion. If our present knowledge provides no plausible mechanism by which A can cause B, can we actually rule it out? John Snow was not able to suggest how polluted water could be the means by which cholera is spread, but even in 1854, there was no good scientific reason for ruling out the possibility that it might be.
(8) Experiment: If we change A, does B change as well? If people stop smoking, does the death rate from lung cancer fall? We now know that it does. Not only do deaths from lung cancer in a population increase when the proportion of smokers goes increases , an individual who gives up smoking reduces his or her chance of contracting the disease depends on the total number of cigarettes smoked . Bradford Hill did include laboratory experiments in his paper, such as the effect of tobacco smoke on dogs, but because he was writing specifically for epidemiologists he considered those to be part of coherence.
(9) Analogy: Are there analogous examples? After it had been established that thalidomide and rubella can produce birth defects, it was easier to make the case that some other birth defect could be caused by a drug or a viral disease.