17.1.         Chapter 17 describes the use of models and estimates to complement the observed data required for the compilation of statistics on the international supply of services, which are frequently needed because (a) part of the required information cannot be collected or can be gathered only at an unsustainable cost, and/or (b) the available sources cannot provide the required coverage, detail, frequency and/or timeliness foreseen by the international standards and/or (c) the various sources to be combined in the collection system are partially overlapping. The chapter contains the following sections: a summary of good practices (section A); imputation for filling data gaps and for data editing purposes (section B); forecasting, back-casting and revising time series (section C); allocating to individual EBOPS items and trading partners (section D); and model-based estimates (section E).