Overview

Consistent with standard statistical practice, the U.S. Energy Information Administration (EIA) has selected and tested a new sample for the Weekly Natural Gas Storage Report (WNGSR). The transition to publishing estimates based on the new sample will occur in two steps. First, using the new sample, EIA will provide revisions to WNGSR estimates for the most recent eight weeks and publish them on Monday, November 18, 2024, outside of the regular publication schedule. These revisions blend the estimates computed from the previous sample and new sample on a graduated basis, explained in detail below. Then, on Thursday, November 21, 2024, EIA will begin publishing estimates based solely on the new sample during the regular publication cycle.

EIA samples are reselected periodically as a standard practice using an established methodology. Although some of the companies selected for the weekly sample will be different in this new sample, the established sampling and estimation methodologies for the WNGSR have not changed. The last sample reselection occurred in 2018.

This report provides information about estimates based on the new sample, compares these estimates with published estimates for working natural gas stocks based on the previous (2018) sample, and details the process for transitioning to the publication of estimates based on the new (2024) sample.

Highlights of this report include:

• Using available data since July 2024, differences between the two sets of estimates of working natural gas stocks based on the new (2024) sample and the previous (2018) sample are small and not statistically significant, at the 5% level of significance.
• The coverage of the new sample is very similar to the coverage of the previous sample in terms of the measure of size that was used to select the sample. For the Lower 48 states, operators in the new sample account for 97% of the total measure of size approximately equaling the coverage for the previous sample.
• EIA will revise the estimates for the eight-week period from September 20 through November 8, using blended estimates of the previous (2018) and new (2024) sample. EIA will also publish revisions to estimated measures of sampling variability for the transition period using the same weights. Working natural gas estimates published on Thursday, November 21, 2024, for the week ending November 15, 2024, will be based solely on data from the new (2024) sample.

Sample selection methodology

EIA survey samples are reselected periodically as a standard practice. The validity of the results from a sample depends on the completeness and accuracy of the frame from which the sample is drawn. The frame for the EIA-912 survey, the Weekly Underground Natural Gas Storage Report, is based on companies reporting data on Form EIA-191, Monthly Underground Natural Gas Storage Report. The Form EIA-191 is completed by all operators of underground natural gas storage fields in the Lower 48 states. Data are provided for storage volumes in each storage field and reservoir. The sample for the EIA-912 survey starts with constructing a sampling frame by aggregating volumes of working natural gas in storage by storage operator and by region, where the South Central region is split by field type into South Central-Salt and South Central-Nonsalt.

Over time, the size and number of the storage fields operated by a given company change, and operators enter or exit the industry. As these changes occur, eventually the initial samples drawn from that frame may no longer be fully representative of the current industry. This situation is addressed most often by selecting a new sample from a more current frame, which better reflects the current industry. Selection of a new sample does not necessarily require or reflect a change in sampling or estimation methodology.

The sampling procedure for the WNGSR uses a stratified sample design with a probability proportionate to size (PPS) sample selection and a measure of size equal to the average between an operator’s most recent October and March monthly working gas stocks at the time of the sample selection . The six strata are based on geographic region and field type (i.e., salt or nonsalt): East, Midwest, Mountain, Pacific, South Central-Salt, and South Central-Nonsalt.

Reported volumes of working natural gas in storage as reported on the Form EIA-191 are aggregated by storage operator and by stratum to prepare the sampling frame for the EIA-912 for each region. For each stratum, the data are divided into two groups. The first—the certainty group—consists of the larger operators in the region as well as all operators with storage fields in more than one stratum. All operators in the certainty group are selected for the sample. The second—the noncertainty group—consists of all other operators, who were selected for the sample with a probability proportional to their size.

Sample selection for 2018

In September 2018, EIA implemented a sample design for WNGSR based on the five-region format for the Lower 48 states. Several changes were incorporated in the sample design, including the collection of a weekly census in the Pacific region, data collection procedures that explicitly separated the salt and nonsalt volumes in the survey submissions from the operators in the South Central region, and a modification to the rounding methodology for published WNGSR estimates. Data collection for the sample began with reports for the week ending February 2, 2018. EIA conducted internal testing and validation of the sample between February and September 2018 before publishing estimates using the sample design. The methodological changes were announced and documented in August 2018 . In addition, EIA published a data history covering the eight-week transition period to help data users provide context for the new data series.

Sample selection for 2024

In May 2024, EIA selected a new sample, in accordance with the established WNGSR sample methodology. EIA selected 88 operators for the new (2024) sample from a sampling frame of 133 operators in the Lower 48 states, compared with 85 operators that were selected for the previous (2018) sample. The net effect of this sample reselection was that ten operators were added, and seven operators were dropped.

Operators in the 2024 sample account for 97% of the total measure of size (average of October 2023 and March 2024 values of working natural gas storage in the Natural Gas Monthly) in the Lower 48 states and range from 95% in the Midwest region to 100% in the Pacific region. Table 1 shows the sample coverage rates in terms of the measure of size by region.

Table 1. Sample coverage rates for the new (2024) sample
Region	Sample coverage rate
East	97%
Midwest	95%
Mountain	97%
Pacific	100%
South Central	97%
South Central (Salt)	97%
South Central (Nonsalt)	97%
Data source: U.S. Energy Information Administration

Comparison of statistics from the previous (2018) and new (2024) samples

The EIA-912 system has collected data from both samples since the week ending July 26, 2024, to allow for comparison and assessment of the data before implementation of the 2024 sample on November 18, 2024. EIA conducted a comparison of WNGSR published estimates using the 2018 sample with estimates based on the new 2024 sample using the monthly data from the EIA-191 survey. As a census survey, the monthly EIA-191 data generally are considered more accurate because they include finalized data adjustments from some respondents and do not include sampling error.

A comparison of data for the Lower 48 states by month shows comparable performance between the weekly estimates under the previous (2018) and new (2024) samples compared with monthly data in each month (Table 2). The average difference of the weekly estimate using the new sample compared with the monthly volumes was only -4 Bcf (billion cubic feet), or -0.10%, and only slightly smaller than the difference from the previous (2018) sample over the period analyzed. These differences are within the target relative standard error of no more than 5% of the working gas estimate.

Table 2. Published monthly data for working gas in underground storage in the Lower 48 states (Lower 48) compared with interpolated estimates from the weekly EIA-912 survey using the previous (2018) and new (2024) samples
Date	Storage volumes			Differences from published monthly data (billion cubic feet [Bcf])		Differences from published monthly data %
	Published Monthly	2018 Sample	2024 Sample	2018 Sample	2024 Sample	2018 Sample	2024 Sample
31-Jul	3,264	3,264	3,269	0	5	0.00%	0.15%
31-Aug	3,367	3,353	3,355	-14	-12	-0.42%	-0.36%
Average	n/a	n/a	n/a	-7	-4	-0.21%	-0.10%
Data source: U.S. Energy Information Administration. Published monthly data in the Differences Between Monthly and Weekly Working Gas in Storage, November 2018; 2018 and 2024 sample estimates are linearly interpolated values based on weekly volumes from the EIA-912 estimation system. Note: The 2018 sample data only includes published weekly estimates. The 2024 sample data also includes unpublished revisions that did not meet the publication revision threshold. Differences are calculated as weekly volumes minus monthly.

The EIA-912 system provided weekly estimates based on the two samples for the period from July 26, 2024, through November 8, 2024. For the Lower 48 states, the average difference between the two samples is 4 Bcf (Table 3), with a maximum of 7 Bcf.

Table 3. Summary statistics comparing weekly estimates of natural gas in storage based on the previous (2018) and new (2024) samples, by region, July 26, 2024, to November 8, 2024 billion cubic feet (Bcf)
	East Region	Midwest Region	Mountain Region	Pacific Region	South Central Region	Salt	NonSalt	Total Lower 48
Average Differences	1	-1	1	2	1	-2	3	4
Average Differences %	0.13%	-0.15%	0.41%	0.58%	0.08%	-0.60%	0.33%	0.10%
Data source: U.S. Energy Information Administration Note: The 2018 sample results are based on published weekly estimates. The 2024 sample estimates reflect the effect of all data submissions, including unpublished revisions of less than 4 Bcf. Differences are calculated as volumes based on 2024 sample minus volumes based on 2018 sample.

Transition to the new (2024) sample

The new sample will be phased in by revising previously published data over eight weeks to transition from the previous (2018) sample to the new (2024) sample. Working natural gas values will be calculated as a weighted average of the estimates from the previous sample and the new sample during the transition period. The published stock values will be calculated as a weighted average of the estimates based on both samples, with each ensuing week using a decreasing weight on the estimates from the previous sample and an increasing weight on the estimates from the new sample (Table 4).

Transition period estimates based on this weighted average combination will be released as revisions to the WNGSR historical database on Monday, November 18. Revisions to estimated measures of sampling variability during the transition period also will be published at this time. Until these revisions are officially published in November, working natural gas stock estimates will be based solely on the previous (2018) sample. Revisions to the historical data also will be noted in the revisions history spreadsheet on the EIA website. The first WNGSR report based entirely on the new sample (2024) will be for the week ending November 15, 2024, which will be published on November 21, 2024.

Table 4. Weights for combination of previous (2018) and new (2024) samples
Week Ending	Weight on estimated stocks for 2018 sample	Weight on estimated stocks for 2024 sample
9/20/2024	7/8	1/8
9/27/2024	3/4	1/4
10/4/2024	5/8	3/8
10/11/2024	1/2	1/2
10/18/2024	3/8	5/8
10/25/2024	1/4	3/4
11/1/2024	1/8	7/8
11/8/2024	0	1
Data source: U.S. Energy Information Administration