What each study type answers and why conclusions do not transfer automatically between levels.
In vitro: testing mechanism
In vitro systems allow controlled examination of binding, signalling or cellular response. They are powerful for defined mechanisms, but simplify biology and depend on the selected cell, concentration and endpoint.
In vivo: an integrated system
Animal models add absorption, distribution, metabolism and organ interactions. They also introduce species differences and greater variability. ARRIVE 2.0 emphasises transparent reporting of design, sample size, randomisation and exclusions.
Clinical evidence: a defined protocol
A clinical trial answers a prespecified question in a defined population, duration and comparator. Even then, generalisation beyond eligibility criteria needs care. For a research-material supplier, clinical evidence provides scientific context, not use instructions.