Most standards are expressed as specification standards which prescribe sizes, dimensions or materials. Performance standards are much more flexible and, therefore, much more desirable in practice, but unfortunately they require a degree of sophistication which may not exist in a particular country. A useful basis for establishing the major urban land uses is given by Robin Best, following the original data requirements for the British new towns, which distinguish four main categories: housing; industry; open space; and education. An important measure in terms of maximising the return on investment in housing land is the proportion of the total area used for actual lots. Obviously a percentage must be used for roads, community facilities and open space, but in low-income areas, where subsidies should be low and where costs must be partially recovered from the beneficiaries, this should be kept to a reasonable minimum.