The Power of Randomized Controlled Trials

In RCTs, study participants are randomly allocated to two or more groups and then assigned to receive an intervention such as HRT or to receive no active treatment (i.e., placebo or to continue with their usual care). RCTs greatly add to the confidence of measured results because the structure of an RCT helps to eliminate many of the inherent biases that are in observational studies. For example, in cohort studies of HRT and CHD, it is hypothesized that women who choose to take HRT are generally healthier and have better healthy lifestyle practices than women who do not choose to take HRT. Because participants in an RCT are randomly assigned to treatment and control groups, they are less likely to have differences in other factors that might prevent or promote heart disease.

Table 8-1 Understanding Study Results

Typical summary rates from randomized, controlled trials:

Incidence rate =

Number of new cases of disease over a defined period Number of persons at risk during the period p. i . . i ,nm Incidence rate among the treated group

v y Incidence rate among the placebo group

Summary measures that may be more meaningful for clinicians:

Attributable risk (AR), or risk difference = (Incidence rate among treated group) -(Incidence rate among placebo group)

Number needed to treat (NNT) or number needed to harm (NNH) =

Reciprocal of AR, or 1 / AR

The decreased likelihood of a healthy user bias in an RCT may explain why HRT appeared to be protective in cohort studies but later proved to be harmful. Because RCTs have this inherent ability to remove many important potential forms of bias (but are not immune to biases themselves), a physician can have more confidence that they reflect the true association between the HRT treatment and CHD outcomes. Despite decades of work, dozens of observational studies, and structured reviews that strongly suggested a protective effect of HRT for CHD, a single, large RCT trumped them all and caused a sudden reversal in physicians' prescribing behavior.

The results of the WHI study, released in 2002, sent a shock wave through the medical community. For the first time, a large, randomized trial showed that HRT—given to otherwise fairly healthy postmenopausal women—caused a statistically significant increase in CHD events. Within days of the release of the WHI primary results, many women called their physicians to decide whether they should continue with HRT. Many physicians drastically changed their prescription of HRT based on the WHI; within 9 months, prescriptions of the most popular formulation of HRT decreased by as much as 61% (Majumdar et al., 2004). Perhaps more than any other single study in modern medical history, the WHI report dramatically changed a widespread, common medical practice.

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