Differences Between Monthly and Weekly Working Gas In Storage
Latest update: July 17, 2015
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 May 2015, estimated total working gas stocks have exhibited an average absolute error of 30 billion cubic feet, or 0.6 percent.
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).
|Table 1. Monthly-Weekly Differences in Underground Storage Estimates (Billion cubic feet)||Download History (2010 to Present)|
|Natural Gas Monthly||Difference|
|Month End||East Region||Midwest Region||Mountain Region||Pacific Region||South Central Region||Total Lower 48||East Region||Midwest Region||Mountain Region||Pacific Region||South Central Region||Total Lower 48|
Sources: Monthly from weekly values-linearly interpolated values based on volumes available with the Weekly Natural Gas Storage Report System; Natural Gas Monthly.
Note: Totals may not equal sum of components because of independent rounding.
Sources of Differences Between the Monthly and the Weekly Series
EIA determined a number of factors that can contribute to the implied differences:
- Inventory reports by respondents to both the monthly and weekly systems can differ somewhat, sometimes simply because activities did not occur in the schedule implied by the linear interpolation of the weekly data. However, these differences historically have tended to be less than 1 Bcf in absolute value per respondent on average.
- Reclassification of base gas to working gas contributed to differences between the weekly and monthly series, owing to the 2-month lag between the two series.
- The estimation methodology may produce weekly volume estimates for the nonsample operators that differ from the reported monthly values.
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.
To some extent, differences between the two data series likely will continue. EIA will provide periodic updates of the information in the tables above as an ongoing resource for reference or for those who may want to modify their own comparative assessment of weekly or monthly storage trends.