In many cases, data for 1990 were not available. In particular, a full analysis of revenues and expenditure for the water sector at city' level was not possible due to incomplete data on sources. Data were collected from respondents on the basis of their knowledge and available sources of information. It lias to be noted that some indicators (e.g. w ater loss, population in households connected to wastewater treatment) are regularly collected in some OECD countries, but a common methodology on these statistics lias proven difficult to establish thus far.