Contractor’s Report
to CalRecycle
39
Appendix A
Methodology and Data Limitations
This appendix briefly summarizes the methodology used for this report, the level of accuracy and
sources of uncertainty, and differences with previous CalRecycle reports.
The market flow estimates presented in Tables 1 and 2 are thought to be accurate to within +/- 10
percent, which may be an upper bound on the potential accuracy of waste tire flow studies
generally.
The estimates cited in this report are based on surveys, interviews, analysis of data in
CalRecycle’s Waste Tire Manifest System and review of written information. Because these
sources are generally incomplete and conflicting, the study team evaluated them for accuracy,
double counting issues, and overall consistency and selected the best available estimate for the
facilities and market categories analyzed.
Data limitations include:
Conversion Factors:
Firms and CalRecycle typically use a standard conversion factor of 20
pounds per tire, even though waste tire weights vary significantly. According to the Rubber
Manufacturers Association, based on national average statistics: passenger tires weigh 22.5
pounds; commercial/truck tires weigh 110 pounds; mixed loads of passenger and light truck
tires average 32.8 pounds per tire; and medium truck tires and off-the-road tires may weigh
hundreds or even thousands of pounds. Manifest data in particular is subject to large errors as
data is allowed to be entered in tons, pounds, number of tires, or cubic yards and conversion
factors may not accurately represent the true amounts, especially when there are mixed loads
of passenger and non-passenger tires. If a truck tire weighing 110 pounds is manifested by
number count, the manifest system does not distinguish between that tire and a 22 pound
passenger tire as both are counted as one 20-pound passenger tire equivalent.
Data Entry:
As one example, CalRecycle estimates that approximately 25 percent of
comprehensive trip log reports have errors.
Un-Manifested Flows and Off-the-Books Transactions:
Some tire flows are not
manifested, either due to CalRecycle-approved exemptions or through failure to submit
required trip logs. Some flows, especially of used tires, are sometimes treated as off-the-
books transactions and are not reported in surveys or tracked by generators, haulers, and/or
processors. Approximately 15 percent of waste tire flows to ports in 2011 were estimated to
not have been recorded and manifested (or recorded as legal weights when containers were
loaded overweight). For the purposes of reporting in this study, the midpoint between
documented export flows and estimated flows (some 15 percent higher) was used for
purposes of tabulation and presentation in graphs.
Discrepancies between Inputs and Output
s
:
Manifest data provides data on inputs to
facilities, while surveys provide data on outputs sent to market uses. Output data is often
based on shipping data or facility estimates that do not reflect stored inventories and that may
occur in a different study year than when the waste tire inputs to make them were received.
This study reports all data on the basis of incoming tire equivalents (i.e., whole tire inputs)
associated with reported product sales and utilizes average yield factors for this conversion
