A derived design number — the thermal resistance L/k of a layer — inherits uncertainty from every input. The stacked bar combines the datasheet’s k tolerance and the thickness tolerance in quadrature into the resistance’s total band. Drag either slider and watch the bar re-balance; the sensitivity habit this teaches — find who owns the budget — is the fastest audit a datasheet ever gets.
Teaching rule only: relative uncertainties of a ratio combine in quadrature when inputs are uncorrelated. Correlations, asymmetric distributions and coverage factors follow the GUM; the simulator teaches the budget-reading habit, not the full machinery.