Likewise, the upper (or ‘superior’) characteristic value Xk,sup is defined as the value of X above which 5% of all results are expected to occur. There is a 95% probability that X will be lower than Xk,sup. Although used less frequently than its lower counterpart, the upper characteristic value is important in situations where underestimating the magnitude of a material property may be unsafe. For example, since the force acting on a retaining wall depends on the weight density of the soil behind it, the wall should be designed to withstand an upper estimate of that weight density. Since the upper characteristic value is not used as often as the lower one, it should always be qualified as ‘upper/superior’ and denoted Xk,sup.