It may be difficult for the contracting authority to accurately anticipate service requirements over the duration of the contract. Contracts typically build in approaches for handling changes to service requirements, in response to changing circumstances (which could also include changing technology). For example the Hong Kong PPP Guide [#131, pages 68-71] describes how changes in circumstance can be dealt with. The South Africa standardized contract provisions [#219, Part K:50] provide for four categories of variation: variations with no additional cost; small works variations; 'institutional' variations (changes in service requirements); and variations requested by the private party.