Two strategies to understand and document causality are: 1) clinical reasoning (CR) and 2) clinical trial (CT). CR identifies: basal state, maneuver and outcome. These components show us the complex of causality, its identification and control allows us to avoid systematic errors, such as: at baseline-improper assembly and susceptibility bias, during the application of the maneuver-performance bias, and at measurement of outcome-detection and transfer bias. In the CT tactics attempt to separate the effect of the main maneuver from the effect of other components that participate in causality previously described in CR. CT takes advantage of its characteristics: the opportunity to manipulate the maneuver and the temporality into the causal relationship. Some features to highlight are: the allocation of the maneuver, blinding of the maneuver, the feasibility of early interruption of the maneuver, the analysis according to maneuver adherence, the groups to compare, the timing of comparative maneuver, and the informed consent. Each occasion the clinician applies all this knowledge and skills, in a conscious way and structured, he improves his efficiency and align medical practice with clinical research.