For a complete and consistent time series, it is necessary to determine the availability of data for each year.
Recalculating previous estimates using a higher tier method, or developing estimates for new categories will be difficult if data are missing for one or more years. Examples of data gaps are presented below:
• Periodic data: Natural resource or environmental statistics, such as national forest inventories and waste statistics, may not cover the entire country on an annual basis. Instead, they may be carried out at intervals such as every fifth or tenth year, or region-by-region, implying that national level estimates can only be directly obtained once the inventory in every region has been completed. When data are available less frequently than annual, several issues arise. First, the estimates need to be updated each time new data become available, and the years between the available data need to be recalculated. The second issue is producing inventories for years after the last available data point and before new data are available. In this case, new estimates should be extrapolated based on available data, and then recalculated when new data become available.
• Changes and gaps in data availability: A change in data availability or a gap in data is different from periodically available data because there is unlikely to be an opportunity to recalculate the estimate at a later date using better data. In some cases, countries will improve their ability to collect data over time, so that higher tier methods can be applied for recent years, but not for earlier years. This is particularly relevant to categories in which it is possible to implement direct sampling and measurement programs because these new data may not be indicative of conditions in past years. Some countries may find that the availability of certain data sets decreases over time as a result of changing priorities within governments, economic restructuring, or limited resources. Some countries with economies in transition no longer collect certain data sets that were available in the base year, or if available these data sets may contain different definitions, classifications and levels of aggregation.
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