Stability margin, defines how far the loads acting on a structure are from their limiting, critical values, at which stability is lost and the load-bearing capacity of the structure is exhausted (see Stability of elastic systems). The ratio of the critical load to the load actually acting on the structure is called the stability safety factor. The choice of an appropriate stability margin is made difficult by the fact that it is impossible to take exact account of a number of factors that affect the magnitude of the critical loads. For example, in the most fully studied case — the loss of stability of a longitudinally compressed rod — such factors are the eccentricity with which the load is applied, the initial curvature of the rod and the inhomogeneity of the material. In calculations for the real working conditions of structures, the influence of additional factors is usually compensated by introducing a correction factor that allows for the probability of defects.