THE PRESENTATION OF EVIDENCE
Last week I told you how evidence about the effectiveness of different medical treatments is obtained. If this evidence is to have any effect on how treatment is applied, it must be presented. This is usually done through scientific medical journals. The way evidence is presented is very important and can greatly influence how patients are treated by the medical profession and others.
Presentation of evidence
It is very easy to be misled by the way in which evidence is presented. Those with an axe to grind, including those funding a study, tend to present their results in the way that is most likely to persuade the reader that the treatment is effective. For instance, studies which are paid for by drug companies are much more likely to have positively presented outcomes than those funded from elsewhere. Also, negative studies are much less likely to reach the light of day than those with positive findings. This is particularly likely to bias the published results of drug trials.
Fortunately, exercise studies are seldom sponsored by commercial organisations, but nevertheless the authors of such studies do sometimes have an interest in the outcomes – be aware of this possible bias.
Absolute and relative benefit
The benefit of a particular intervention is usually expressed in terms of change in risk or outcome such as death. This may be presented as a change in relative risk or a change in absolute risk. A change in relative risk usually makes the treatment seem much more attractive than a change in absolute risk.
Absolute risk of a disease is the risk of developing the disease or a particular outcome over a period of time. It may be expressed as a fraction – say, 1 in 20; as a percentage – in this case 5 per cent; or as a decimal – in this case 0.05.
Relative risk is used to compare the risk in two different groups of people. For example, the groups could be smokers and non-smokers.
All sorts of groups are compared to others in medical research to see if belonging to a particular group increases or decreases your risk of developing certain diseases. For instance, research has shown that smokers have a higher risk of developing heart disease compared to (relative to) non-smokers.
Let us say that the absolute risk of developing heart disease is 4 in 100 (4 per cent) in non-smokers, but the relative risk of the disease is doubled in smokers – to 8 per 100. If the two groups are compared, the increase in risk brought about by smoking is 4 in 100. For every 100 smokers, 4 more individuals will develop heart disease compared to non-smokers. This sounds as bit less impressive than a doubling of risk.
When comparing the results of treatments or other interventions, how the outcomes are expressed can have a big effect on how good the treatment looks. The less common the condition, the truer this is.
Look at the example of the risk of a heart attack and how this can be reduced by taking a particular drug. The risk of a heart attack over the next 10 years in a group of women aged between 40 and 50 may be, say, 1 in 100 – 1 per cent. If taking the drug in question reduces the risk to 1 in 200 (0.5 per cent) it may be reported that the risk of a heart attack, the relative risk, was halved by the drug. However, the absolute risk is a reduction from two deaths to one death for every 200 women – an absolute reduction of 1 in 200. In other words, 200 women would have to take the drug for 10 years to prevent one new heart attack – which seems rather less impressive than halving the risk. Sometimes the effectiveness of a treatment is then expressed as ‘number needed to treat’, i.e. the number of people who need to take the treatment for just one person to benefit – in this case 200.
Mortality rates are often used as outcome measures to compare the efficacy of different drugs and other treatments. Since the ultimate mortality for any treatment regime is 100 per cent – we all die in the end – ‘mortality’ when used for this end has to be qualified. There are two ways of doing this:
- Mortality is expressed as the death rate over the period of study and compared between the groups being studied.
- The death rate of the group being studied is compared with the known death rate of the whole population of the same age and gender – it is usually expressed as deaths per 1,000 persons per year.
Evidence is necessary – but not always easy to translate
Facts unsupported by evidence should be questioned but even when evidence seems to support the facts you must be alert to the possibilities of bias and error.
Very little evidence is so absolute that it is unarguable – but evidence is a great deal better than any other way of reaching the truth, about exercise as about anything else.
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