Observational studies: why are they so important?

Abstract

Randomized clinical trials (RCTs) are known to be the gold standard for medical research, and this study design is preferred for investigating the efficacy of new interventions. Many authors and editors believe that the RCT design always surpasses other research designs. However, this is not an unquestionable truth.The main advantage of RCTs is that they provide better control over possible bias through randomization and blinding. In other words, the great strength of an RCT is its high internal validity. On the other hand, rigid design control could reduce the ability to generalize the results.1 Another issue relating to RCTs is the fact that recruitment, randomization and blinding are not always possible because of technical issues (e.g. surgical procedures) or ethical issues (e.g. in a hypothetical trial to prove the association between smoking and lung cancer, it would be unethical to randomly assign a group to smoke).But what should be done when RCTs are unfeasible or unethical? The answer could be a well-designed observational study. This term describes a range of study designs that includes cross-sectional, case-control, prospective and retrospective cohort studies. The main characteristic of observational studies is that any intervention is not determined by the protocol, but by clinical practice. Cross-sectional studies are used to estimate prevalence. They are relatively quick and cheap, and can be used to study multiple outcomes. However, they do not differentiate between cause and effect or within the sequence of events. They are useful for identifying associations that can then be more rigorously studied using a cohort study or randomized controlled study. The most important problem with this type of study is in differentiating between cause and effect from a simple association. Case-control studies are considered to be simple to organize and useful for hypothesis generation. They retrospectively compare two groups in order to identify predictors of an outcome.They allow calculation of odds ratios. The disadvantages are that they can only evaluate one outcome and that the presence of bias is usually high and difficult to assess. The major difficulty could lie in determining an adequate control group. Cohort studies are excellent for estimating the incidence and natural history of a condition.They may be prospective or retrospective and sometimes two cohorts are compared. They analyze predictors (risk factors) that enable relative risk calculation. Since they measure events in temporal sequence, they can distinguish causes from effects. Retrospective cohorts are considered to be cheaper and quicker, but may have fragile results, particularly if the database is inadequate. Prospective cohorts are more accurate and, with a good protocol (with adequate sample size and follow-up) can have results that are as reliable as those of RCTs. 3The main problem in observational studies is the presence of confounders and selection bias (which are prevented in RCTs through randomization and blinding). A confounder ...

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