Accuracy and reliability
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Short Term Statistics, Business StatisticsHeidi Sørensen
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The statistics is assessed to be reliable as the main series are based on financial information from the reporters (ticket sale). Furthermore the reporters are obliged to monitor the number of passengers and vehicles onboard.
There is few and minor revisions and few errors are found in reported data.
The smallest ferry lines are not included in the statistics. Usually this is lines to the smallest Danish islands or crossing fjords.
In the main series uncertainty is largest for goods by goods motor vehicles that in some cases are estimated based on the number of goods motor vehicles.
Overall accuracy
The statistics are in general reliable, taking due regard to the sampling error.
The overall accuracy on passengers is within a 2 percent range while the accuracy on goods is assessed to be larger among other things because the amount of goods in some cases are imputed.
The main reasons to less accuracy is
- the imputation of goods where the ferry lines does not weigh the transferred vehicles
- missing information on transferred bicycles
There is no sampling error since response rate is 100 percent.
Sampling error
Not relevant for these statistics.
Non-sampling error
The smallest ferry lines are not included in the statistics. They are assessed to cover less than 2 percent measured by number of passengers.
Partly missing data can occur where the data is not registered separately by the ferry lines, e.g. bicycles.
Imputation of missing data are done on amount of goods i cases where the ferry lines does not weigh the road transport vehicles. The imputation is done based on the number of road transport vehicles transferred.
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
There has not been made a quality measurement of the statistics.
The statistics is assessed to be of good quality since it covers all major ferry lines and the response rate is high. On specific subcategories the quality might be lower.
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. Data are revised 4-8 quarters back in connection with dissemination of new quarterly data.
The revisions are usually of minor importance.