Accuracy and reliability

Overall accuracy

It is not possible to give a quantitative assessment of the precision of the forest accounts as a whole. The forest accounts are based on several sources, each of which has its uncertainties. In the compilation of the accounts, these uncertainties are continued. However, consistency checks are carried out through the cross-matching of sources, which means that the primary sources are corrected if they lead to impossible results (e.g. negative gross increment).

The sizes of the physical stocks (forest area, growing stock) are considered reliable and reasonably accurate, seen over a number of years. It is not possible in practice to carry out measurements in and of all the forests on January 1, and the changes from one year to the next may therefore reflect measurement errors and other uncertainties. However, seen over a period of time, it is reasonable to assume that the order of magnitude of the numbers and the development are accurate.

It is important to point out that the parts of the accounts which relate to the size of the physical stocks are more precise than the monetary accounts. This is because the monetary accounts are based on a number of assumptions on the estimated prices as well as inherent in the method used. The estimate of the monetary value of the growing stock is therefore very sensitive to changes in several assumptions.

Sampling error

The forest accounts are not directly based on a sample. Thus, a sampling error for the accounts cannot be estimated. However, the main source for the accounts concerning the growing stock is the National Forest Inventory, which is sample-based. The statistics on felling of wood in the Danish forests is also sample-based, see documentation of these under 'Source data'.

Non-sampling error

The uncertainties in the forest accounts are described separately for each of its three parts, which each have different sources and methods, and thus different estimations of uncertainty.

The primary source of uncertainty for the forest area is that the regional distribution is calculated from data from the National Forest Inventory, which does not have the same definition of forest area. This can affect the accuracy of the forest area in different regions, to the extent that there are regional variations in how these definitions affect the calculated forest area. The annual change in forest area up until 2010 is calculated using linear interpolation and can therefore only be taken as average sizes.

Growing stocks in physical units are calculated from data collected at sample plots in the forests. Uncertainties come both from the sample size, and from the calculation of growing stocks from measurements of the trunk diameter, the height of the tree, and the tree species. For further information, see the documentation of the National Forest Inventory under 'Source data'. In general, the uncertainty is greater at the detailed level. There is also uncertainty connected to the time at which measurements were performed, which might be throughout the year, and not on January 1 every year. It is therefore best to look at developments over multiple years instead of interpreting the development in a single year in isolation.

All sources of uncertainty in the physical accounts of the growing stock are also reflected in the monetary accounts, since the former is used to compile the latter. In addition, there are multiple sources of uncertainty specifically for the monetary accounts. The valuation method is based on multiple assumptions and estimates, such as estimates of yearly timber prices for different tree species and average loss during tree felling. Valuation of growing stocks is done using the so-called stumpage value method, which is described by the FAO.

Quality management

Statistics Denmark follows the recommendations on organisation and management of quality given in the Code of Practice for European Statistics (CoP) and the implementation guidelines given in the Quality Assurance Framework of the European Statistical System (QAF). A Working Group on Quality and a central quality assurance function have been established to continuously carry through control of products and processes.

Quality assurance

Statistics Denmark follows the principles in the Code of Practice for European Statistics (CoP) and uses the Quality Assurance Framework of the European Statistical System (QAF) for the implementation of the principles. This involves continuous decentralized and central control of products and processes based on documentation following international standards. The central quality assurance function reports to the Working Group on Quality. Reports include suggestions for improvement that are assessed, decided and subsequently implemented.

Quality assessment

The physical part of the forest accounts is based on and collects other statistics on forest area and growing stock, and thus has at least the same good quality as the corresponding sources. However, it is important to point out that even though the accounts are compiled as a time series of years, it is not possible in practice to measure the forests on a specific date. Therefore, changes in stock between single years should be interpreted with caution. Instead, the accounts should rather be looked at over a time period.

Concerning the valuation of the growing stock (the monetary part), there are great uncertainties and the method used is based on a number of assumptions and estimates. This applies not least to the estimation of prices. Thus, it should be considered experimental statistics rather than a definitive estimate.

Data revision - policy

Statistics Denmark revises published figures in accordance with the Revision Policy for Statistics Denmark. The common procedures and principles of the Revision Policy are for some statistics supplemented by a specific revision practice.

Data revision practice

The forest accounts are revised 2 years back with each publication of a new year as this allows using the final version of all data sources. Extraordinary revisions may be done if the sources (e.g. the National Forest Inventory) are revised significantly, to ensure consistency over time.