Target Trial Emulation in Observational Research-Strengths, Limitations, and Methodological Considerations.

Cohort studies are vulnerable to confounding and time-related biases. Recent advances emphasize the importance of emulating a hypothetical target trial to minimize several of these biases. The target trial framework is a critical tool for designing cohort studies. It requires explicit specification of protocol components: eligibility criteria, treatment strategies, outcomes, timing of follow-up (time zero), and the causal contrast to be estimated. Critical to this framework is defining "time zero" when treatment assignment takes place and follow-up begins, which prevents prevalent user and immortal time bias and ensures proper temporal sequencing of confounders, exposure, and outcome. By guiding design, data collection, and analysis, the target trial framework helps align observational studies with the principles of randomized trials, improving the validity and interpretability of their findings. While it cannot eliminate unmeasured confounding, this approach promotes transparent, bias-conscious cohort design, minimizes time-related bias, and supports more robust causal inference with observational data.