Bridging national and reference definitions for harmonizing forest statistics

Harmonisation is the process of making information and estimates comparable across administrative borders. The degree to which harmonisation succeeds depends on many factors including the conciseness of the definitions, the availability and quality of data, and the methods used to convert an estimate according to a local definition to an estimate according to the reference definition. Harmonisation requires the availability and use of common reference definitions and methods for converting from estimates based on national definitions to estimates based on reference definitions. This article focuses on conversion methods which are characterised as ‘bridges’ because they can be seen as means of crossing from islands of local definitions to the mainland of a reference definition. A structured approach is proposed for constructing bridges of three kinds: reductive, neutral, and expansive bridges. A hierarchical decision tree is presented to guide users and to summarise the propositions, and case examples with different types of bridges illustrate the concepts. Although the article addresses harmonisation of forest information, the results are relevant for harmonising a broad variety of area statistics.