A Feasibility Study brings together the resource model, test work, engineering, geotechnical conditions, infrastructure, environmental and permitting inputs, execution planning and the financial model on which the Board decides. Data and assumptions sit beneath that work. The data is what has been observed, measured, tested or priced: drilling, metallurgical test work, geotechnical investigation, engineering quantities and quotations. The assumptions are what the study team adopts where evidence runs out. Every study contains both. Successive phases reduce uncertainty to a level appropriate for the decision. The quality of the data governs the range within which the study can reasonably be right. A financial model can calculate weak inputs precisely. It cannot make them reliable. We see the consequences during independent reviews. A mining method may be selected on insufficient geotechnical information. Process performance may rely on unrepresentative test work. Design may advance before the required investigation or engineering definition is complete. The recommendation is often the same: collect and validate data before advancing. That is why project phases have different minimum standards. Scoping may rely on broad assumptions. Prefeasibility should test the alternatives and establish the preferred case. By Feasibility Study, material assumptions should be supported by evidence consistent with the claims presented. A study advanced on data below the required threshold claims confidence it has not earned. Assumption traceability is a basic study control. Metallurgical recoveries should trace to representative test work. Capital quantities should trace to engineering documents. Operating costs should trace to supplier information, operating data or defensible benchmarks. Where hard evidence is unavailable, the study should state the basis, uncertainty and treatment. Project failure often takes root early, in assumptions not tested while the owner still had time to change them. The mechanism is frequently optimism. The ramp-up is too steep. Grade or recovery is read too generously. Productivity is assumed without sufficient evidence. Contingency supports confidence the scope has not earned. For capital cost, the control lies in scope maturity, estimate discipline and honest treatment of risk and contingency. For inputs the project cannot control, particularly commodity prices, the economics should be tested across credible downside conditions agreed with decision-makers. A credible feasibility case shows whether the project remains acceptable when important assumptions move against it. Testing that before capital is committed is a central purpose of study governance, independent peer review, and the Capital Investment System (CIS). The study team develops the case. Governance tests it. The Board decides.