<ns3:p><ns3:bold>Background</ns3:bold>: Precision medicine is the Holy Grail of interventions that are tailored to a patient’s individual characteristics. However, conventional clinical trials are designed to find differences with the implicit assumption that the effect is the same in all patients within the eligibility criteria. If this were the case, then there would be no grounds for treating different patients differently. One consequence of the assumption is that the variance in the outcome variable would be the same in treated and control arms. We reviewed the literature to see if this were the case and so to see how often precision medicine would not be useful.</ns3:p><ns3:p> <ns3:bold>Methods</ns3:bold>: We reviewed parallel trials with quantitative outcomes published in 2004, 2007, 2010 and 2013. We collected baseline and final standard deviations of the main outcome. We assessed homoscedasticity by comparing the variance of the primary endpoint between arms through the outcome variance ratio (treated to control group).</ns3:p><ns3:p> <ns3:bold>Results</ns3:bold>: The review provided 208 articles with enough information to conduct the analysis. One out of seven studies (n = 30, 14.4%) had statistically different variances between groups, leading a non-constant-effect. The adjusted point estimate of the mean outcome variance ratio (treated to control group) is 0.89 (95% CI 0.81 to 0.97).</ns3:p><ns3:p> <ns3:bold>Conclusions</ns3:bold>: We found that the outcome variance was more often smaller in the intervention group, suggesting that treated patients may end up pertaining more often to reference or “normality” values and thus would not require further precision medicine. However, this result may also be compatible with a reduced effect in some patients, which would require studying whether the effect merits enduring the side effects as well as the economic costs. We have shown that the comparison of variances is a useful but not definitive tool to asses if the design assumption of a constant effect holds.</ns3:p>