D. Use of models and estimates
12.12. The present Guide recognizes that available data sources have their limitations and, therefore, that compilation must include the use of models and estimates. Those limitations include the unsustainable cost of collecting some of the information, the inability of the available sources to provide the required coverage, detail, frequency and/or timeliness foreseen by international standards and the partial overlap of the various sources to be combined in the collection system.. Chapter 17 is devoted to those issues and covers such topics as imputation for filling data gaps and for editing data, forecasting and compensating for the lack of timeliness of data sources, back-casting and revising time series, allocating all services transactions to the relevant EBOPS services categories and appropriate trading partners, and model-based estimates.