In this article, various designs of clinical trials used to obtain new scientific knowledge in the field of clinical medicine are considered from the position of their evidential value in studying the cause‑and‑effect relationship between the influencing factor and result of its potential effect on human health. Basic differences between observational and experimental trials, their limitations due to peculiarities of design of clinical trials are being discussed. A conclusion is made that validity of results of clinical trials should be assessed taking into account the limitations that are typical of various designs. Accuracy of clinical trials depends on many factors that can distort the obtained results as compared with true values. It is noted that observational trials are subject to systematic and accidental errors to a greater extent than experimental ones. It occurs because design characteristics do not allow observational trials to control the mistakes associated with possible incompatibility of comparison groups. They can detect a statistical relation between the phenomena, but only randomized clinical trials can prove that there is a causal relationship. Accuracy of a randomized clinical trial can be increased using systematic reviews and meta‑analysis.