Stiffness can be determined from a load versus deflection curve by calculating the slope of the curve, though nonlinearity may complicate this. The initial slope can provide a basic stiffness value, but for more accurate results, an average stiffness over a specific deflection range is often recommended. This average stiffness is represented by the slope of a chord connecting two points on the curve, rather than a single point. The choice of stiffness definition may depend on the intended application and the nature of the load-deflection relationship. Understanding these nuances is essential for accurate stiffness evaluation.