Formulaic updating, on the basis for example of a moving average of past prices, reduces the potential for stabilisation and hence the value of the intervention, but does offset the costs of intervention. However, the buffer stock can only sell what it has previously bought so once its stock is exhausted the authority has no further means of defending the ceiling. The consequence is that buffer stock agreements tend to [be] more effective in limiting price falls than in curtailing the incidence and magnitude of spikes. Although attacks can take place either on a floor or a ceiling price, the problem is more serious at the ceiling.