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Data Collection Procedures
Every Monday, retail
on-highway diesel prices are collected by telephone and fax from a sample of
approximately 350 retail diesel outlets, including truck stops and service
stations. The data represent the price of ultra low sulfur diesel (ULSD) which
contains less than 15 parts-per-million sulfur and low sulfur diesel (LSD) which
contains between 15 and 500 parts-per-million sulfur. EPA requires that all
on-highway diesel sold be ULSD by December 1, 2010 (September 1, 2006 in
California). ULSD was projected to be available at many retail locations by
October 15, 2006 (September 1, 2006 in California). Historically, EIA collected
the price of on-highway fuel without distinguishing the sulfur level. In January
2007, the weekly on-highway diesel price survey began collecting diesel prices
for LSD and ULSD separately. On February 19, 2007 EIA began releasing the
detailed sulfur price information that was determined to be accurate. On
December 8, 2008, the publication of LSD Prices at the U.S. level was
discontinued. The LSD price estimates at the U.S. level no longer met accuracy
requirements due to the diminishing number of stations selling LSD. EIA will
continue to collect LSD prices from the retail outlets in our sample that still
sell this fuel type. However, the LSD prices will only be included in the
average prices of all types of diesel. Prices for Average All Types will
continue to be published until the LSD fuel diminishes to a level so low that
the All Types and ULSD average prices are equal. Prices for ULSD will continue
to be published.
The prices
reported in this survey are subjected to automated edit checks during data
collection and data processing. Data flagged by the edits are verified with the
respondent. Imputation is used for companies that can not be contacted. The
average survey response rate for 2006 was 99.1%.
The price estimates for ULSD and LSD each week are obtained using simple
averages at the sampling cell level. For publication regions that constitute a
combination of sampling cells, the volume of on-highway diesel sold in that
region as reported in the EIA-782 monthly survey is used to weight the sampling
cells and obtain publication level prices for ULSD and LSD. The average price of
all types of diesel is calculated by weighting the average prices of ULSD and
LSD according to the number of outlets in each area selling each of the products
as reported for that week in the weekly on-highway diesel price survey.
The average prices are released by 5:00 P.M. Monday, except on government
holidays, in which case the data are released on Tuesday (but still represent
Monday's price). These data are made available through EIA’s hotline
(202-586-6966), EIA’s web page, and through EIA’s E-mail notification, regular
and wireless.
Statistical Methodology
The sample design for the weekly diesel price survey was a
two-phase design. The first phase constituted construction of a frame of 2,207 company-State
units (CSUs) from the combination of two sample cycles of the EIA-782A and
EIA-782B surveys that collected monthly petroleum products’ sales at the State
level. For sampling purposes, any combination of State and company where diesel was
sold through retail outlets as reported on the EIA-782 surveys defined a CSU,
the sampling unit.
For the second phase, a sub-sample of the 2,207 CSUs from
phase 1 was selected using probability proportional to size (PPS). The measure
of size for each of the two sample cycles separately was normalized using the
annual State sales’ volumes from the monthly survey divided by the unit's
probability of selection in the monthly survey. The two cycles’ normalized measures were then added to form one size.
Each CSU in the frame, therefore, had a size,
and the sum of the CSU sizes within each sampling cell equaled the allocation for
each cell. Within the second phase was a
second stage to identify and select the actual outlets reporting for the
company. This identification was done by
contacting the sampled companies and asking them to provide the names,
addresses and telephone numbers for truck-stops and retail outlets selling
diesel fuel for each sampled CSU. Additionally,
they were also asked to provide the proportion of diesel fuel sold through
truck-stops for each sampled CSU. This
proportion was used to preserve and control the number of truck stops and
retail stations selected for each CSU.
Sample allocations for each sampling cell were calculated
using the average standard errors across reporting periods for the previous
year of weekly diesel fuel survey prices for each of the sampling cells. The
sample size was determined for each cell by the formula: n'
= (e/t)2 n, where t was the
targeted standard error, n was the previous sample size for the cell, e was the
average of the previous sample's weekly standard errors, and n' was the new
sample cell allocation. In addition, a second allocation based on proportional
representation within the next larger aggregation cell to which the original
sampling cell would contribute was also obtained. For example, the PADD IB sampling cell
contributes to the PADD I cell. The maximum of the two allocations for each cell
was then designated as the cell allocation for the sampling cell. The sample was targeted at the time of
selection to yield standard errors of 1 cent for the U.S.,
the East Coast, the Midwest, and the Gulf Coast. The remaining areas were targeted at the 1.5 cent level. These targeted standard
errors were based on the price levels that were realized at the time of
selection which translated the errors to roughly a 1%/1.5% Coefficient of
Variation, respectively. This allocation
procedure yielded a targeted sample size of 350 truck stops and retail outlets
for the diesel fuel survey.
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