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Different series numbers (column “Series”) are used to store different time-series versions of national accounts statistics. Series numbers with two digits (10,20) refer to data compiled following the 1968 SNA national accounts methodology, while series numbers with three digits (100, 200, etc) refer to data compiled using the 1993 SNA national accounts methodology, and series numbers with four digits (1000, 1100, etc) refer to data compiled using the 2008 SNA national accounts methodology. In addition to different methodologies, different series numbers are used when data are reported in different currencies, fiscal years, or by different sources. Furthermore, data are stored under a new series number whenever there are significant changes in compilation practices which make the time series no longer comparable.