Differences Between Monthly and Weekly Working Gas In Storage |
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Latest update: December 31, 2008 Note: The weekly storage estimates are based on a survey sample that does not include all companies that operate underground storage facilities. The sample was selected from the list of storage operators to achieve a target standard error of the estimate of working gas in storage which was no greater than 5 percent for each region. Based on a comparison of weekly estimates and monthly data from May 2002 through October 2008, estimated total working gas stocks have exhibited an average absolute error of 18 billion cubic feet, or 0.8 percent. Background The Energy Information Administration (EIA) provides weekly estimates of working gas volumes held in underground storage facilities at the national and regional levels. These are estimated from volume data provided by a sample of operators that report on Form EIA-912, "Weekly Underground Natural Gas Storage Report." The EIA first released its estimates of underground storage for the week ending May 3, 2002, on May 9, 2002, in the Weekly Natural Gas Storage Report (WNGSR). EIA also publishes data for the end-of-month working gas volumes that it collects from the form, EIA-191, "Monthly Underground Gas Storage Report." While these surveys both result in estimates of working gas in storage, some differences between the two series occur, which are useful to understand when comparing current weekly data estimates with values in prior years or to the corresponding data from the monthly series. Table 1 presents the difference between estimates of working gas in underground storage as reported at the end of each month on Form EIA-191 (a monthly census of all underground storage operators) and end of month estimates interpolated from the weekly estimates (based on a sample of storage operators reporting on Form EIA-912).
Sources: Monthly from weekly values-linearly interpolated values based on volumes available with the Weekly Natural Gas Storage Report System; Natural Gas Monthly. Sources of Differences Between the Monthly and the Weekly Series EIA determined a number of factors that can contribute to the implied differences:
EIA has used a number of estimation methodologies, all of which have been based on using attributes of the reported information from the operators in the sample to estimate working gas volumes for the non-sample companies. There have been periodic changes to the methodology or sample since 2002 that have impacted the resulting estimates. The initial methodology relied on fixed ratios and a fixed sample that may not have captured a change in the relative significance of sample operators and nonsample operators in total inventory in a given period. In October 2003, EIA implemented several adjustments to the weekly series that affected the observed differences beginning in July 2003. These included: a revised estimation methodology, an expanded sample, and revisions for the 17-week period from July 4, 2003 through October 24, 2003. In August 2005, EIA implemented a change to the basic methodology and revised the sample (see Methodology). In conjunction with those changes, revised estimates for the period from mid June 2005 to August were released. |