Free How to Read a Paper: The Basics of Evidence-Based Medicine by Trisha Greenhalgh Page A

findings of your trial will be more relevant to that real world when the trial is finished [14]. See also section ‘What information to expect in a paper describing a randomised controlled trial: the CONSORT statement’ where I introduce the Consolidated Standards of Reporting Trials (CONSORT) statement for presenting the findings of RCTs.
    Cohort studies
    In a cohort study, two (or more) groups of people are selected on the basis of differences in their exposure to a particular agent (such as a vaccine, a surgical procedure or an environmental toxin), and followed up to see how many in each group develop a particular disease, complication or other outcome. The follow-up period in cohort studies is generally measured in years (and sometimes in decades) because that is how long many diseases, especially cancer, take to develop. Note that RCTs are usually begun on people who already have a disease, whereas most cohort studies are begun on people who may or may not develop disease.
    A special type of cohort study may also be used to determine the prognosis of a disease (i.e. what is likely to happen to someone who has it). A group of people who have all been diagnosed as having an early stage of the disease or a positive screening test (see Chapter 7) is assembled (the inception cohort) and followed up on repeated occasions to see the incidence (new cases per year) and time course of different outcomes. (Here is a definition that you should commit to memory if you can: incidence is the number of new cases of a disease per year, whereas prevalence is the overall proportion of the population who suffer from the disease.)
    The world's most famous cohort study, whose authors all won knighthoods, was undertaken by Sir Austen Bradford Hill, Sir Richard Doll and, latterly, Sir Richard Peto. They followed up 40 000 male British doctors divided into four cohorts (non-smokers, and light, moderate and heavy smokers) using both all-cause (any death) and cause-specific (death from a particular disease) mortality as outcome measures. Publication of their 10-year interim results in 1964 [15], which showed a substantial excess in both lung cancer mortality and all-cause mortality in smokers, with a ‘dose–response’ relationship (i.e. the more you smoke, the worse your chances of getting lung cancer), went a long way to demonstrating that the link between smoking and ill health was causal rather than coincidental. The 20-year [16], 40-year [17] and 50-year [18] results of this momentous study (which achieved an impressive 94% follow-up of those recruited in 1951 and not known to have died) illustrate both the perils of smoking and the strength of evidence that can be obtained from a properly conducted cohort study.
    Given here are clinical questions that should be addressed by a cohort study.
     
Does smoking cause lung cancer?
Does the contraceptive pill ‘cause’ breast cancer? (Note, once again, that the word ‘cause’ is a loaded and potentially misleading term. As Guillebaud has argued in his excellent book ‘The Pill…’[19], if a thousand women went on the oral contraceptive pill tomorrow, some of them would get breast cancer. But some of those would have got it anyway. The question that epidemiologists try to answer through cohort studies is, ‘what is the additional risk of developing breast cancer which this woman would run by taking the pill, over and above the baseline risk attributable to her own hormonal balance, family history, diet, alcohol intake, and so on?’.)
Does high blood pressure get better over time?
What happens to infants who have been born very prematurely, in terms of subsequent physical development and educational achievement?
    Case–control studies
    In a case–control study, patients with a particular disease or condition are identified and ‘matched’ with controls (patients with some other disease, the general population, neighbours or relatives). Data are then collected (e.g. by searching back