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Accuracy and reliability

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Short Term Statistics, Business Statistics
Heidi Sørensen
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Maritime Transport over Danish Ports

Maritime statistics are based on censuses among all goods handling ports. The majority of data stems from the quarterly reports from all major ports. The data from the remaining minor ports are summarised annual data. On the main variables there is full coverage and accurate within 3 percent. Minor revision occur without systematic bias.

Overall accuracy

The statistics are for the most part based on monthly reports on individual port calls in the major ports and for the remaining on annually summary data. On the main variables coverage is 100 percent.

Accuracy on the main variables is assessed within 3 percent, i.e. the true amount of handled goods is within +/- 3 percent interval of the published data.

Sampling error

Not relevant for these statistics.

Annual statistics is based on a census of goods transport in the Danish ports with complete data from all ports and ferry lines. There is thus no sampling error on the annual statistics.

In the quarterly statistics data is reported from all population consisting of ports with a cutt-off on goods through-put on 1 mill. tonnes annually. It is not a sample and there is therefore no sampling error.

Non-sampling error

Compilation of statistics by flag state, and by country of loading/unloading are only based on reports from the major ports. In 2012 those ports had 83 percent of the goods unloaded, 89 percent of the goods loaded and 76 percent of the port calls of cargo ships in Danish ports. On the variables weight of goods and (national) tonne-kilometers the coverage is close to 100 percent.

Occasionally port calls with goods occur on the smallest ports not included at all. It is assessed to be of very modest extent.

There are no sampling errors as the statistics are compiled on the basis of censuses.

Measuring errors might occur in the parts of the statistics that ports collects only for statistics use, i.e. type of vessel, type of goods and ports of origin and destination.

In ferry goods a calculation error can occur when imputing weight of goods in cases where only the number of goods vehicles are reported.

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 overall quality assessment is high. The coverage is high with 100 percent response rate and very few data revisions.

On the main variables, amount of goods and national transport performance (tonnes-km) the quality is high and fully covered.

To the extent that small ports without regular goods transport occasionally handles goods, the statistics is underestimated. Based on information from partner ports in Denmark, at least some transactions with the smallest ports can be identified and the underestimation is assessed to be less than 0.1 percent of total tonnes handled.

On specific variables accuracy is less due to the ports limited access to information on type of goods, type of vessel and ports of origin and destination.

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

Preliminary figures are published and revisions occur. Figures are revised 4-8 quarters back in connection to the quarterly disseminations.

Revisions are usually of minor significance.