Reference Sample Program (RSP)

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RSP Schedule
  2025 Schedule
  2024 Schedule
  2023 Schedule
  2022 Schedule
  2021 Schedule
What is the RSP?
Laboratory Participation Requirements
Test Method Selection
Fees and Sample Delivery
Testing and Reporting
RSP Results Evaluation
Unacceptable Results and Retesting
Rating Letter and Final Report
Maintaining Laboratory Accreditation
Program Links and Documents
Interlaboratory Studies (ILS)

RSP Schedule

2025 Schedule

2024 Schedule

2023 Schedule

2022 Schedule

2021 Schedule

*Enrollment cutoff: A late fee may be assessed to those labs opting in to participate past the ‘Enroll by’ date. See Fees and Sample Delivery section.

What is the RSP?

As an integral part of 23 CFR 637 B (see Caltrans IAP), the RSP is a materials laboratory proficiency testing program that uses reference or control samples (samples of known characteristics) to evaluate equipment, testing practices, technician competence, and the accuracy and reproducibility of test results.

To maintain Caltrans accreditation, laboratories must participate in all RSP rounds involving a test method the laboratory carries active accreditation. Failure to participate in a given round of RSP may result in suspension of the laboratory accreditation of that test method.  Laboratories that do not carry accreditation in any of the test methods identified for the RSP evaluation are exempt from this requirement.

For each test method identified for RSP, Caltrans IAP will contact accredited laboratories by e-mail and request participation.  To participate, the laboratory must follow the enrollment instructions provided in the announcement e-mail. Please see Fees and Sample Delivery section for fees associated with preparing and delivering samples.  Delivered samples will have complete testing instructions and a deadline by which the results must be reported.

Once the RSP for a given test method is completed and all test results have been submitted to the Caltrans IAP, a rating letter will be issued to participating laboratories.  From this rating letter, individual laboratories may review their performance relative to the entire population of RSP-participating laboratories for the specific test method.

Laboratory Participation Requirements

Caltrans IAP requires accredited laboratories to participate in the RSP on the test methods selected for proficiency testing.  Laboratories that do not carry active accreditation in any of the test methods identified for the RSP are exempt from this requirement.  To participate in the RSP, laboratories must respond to e-mail announcements and follow the enrollment procedures provided.

As part of the enrollment, laboratory managers must verify the laboratory information by visiting the Statewide Independent Assurance Database and submit a completed RSP Questionnaire (TL-0119) form to Caltrans.Reference.Sample.Program@dot.ca.gov to request updates. For existing laboratories, the RSP Questionnaire serves as process to identify any changes in laboratory information such as lab manager, physical address and/or shipping address, or general contact information. For new laboratories, a unique laboratory identification number (RSP #) will be assigned. The section for requesting accreditation in the RSP Questionnaire does not need to be completed to update laboratory information.

Late Enrollment: Laboratories confirming participation within 30 days after the enrollment deadline will be assessed a late fee. Failure to confirm participation 30 days after the enrollment deadline may result in suspension of the laboratory’s accreditation for that test method.

Test Method Selection

Each year, Caltrans evaluates laboratory performance on test methods regularly used to evaluate soil, aggregate, hot mix asphalt, or cement concrete. Test methods may be selected based on a variety of reasons, such as, frequency of use, variabilities associated with test method procedures, comparisons to established precision and bias statements, or ensuring as many labs as possible have the opportunity to participate. Typically, the test method selected will have an adequate number of accredited laboratories such that a reasonable data set will be produced for evaluating individual lab results.

Fees and Sample Delivery

Laboratories are assessed a flat fee commensurate with the cost of preparing and shipping samples to participate in a given round of proficiency testing. Payment instructions are provided in the announcement email and fees must be paid prior to receiving samples. Public agency laboratories are exempt from fees.

  Standard fees:

• Enrollment fee: $250
• Late enrollment fee: $750

Mailing address for sending checks for the enrollment fee:  Caltrans METS, C/O Reference Sample Program, 5900 Folsom Blvd, Sacramento CA 95819

Upon receipt of fees, Caltrans IAP will ship samples via parcel delivery carrier or IA staff will hand deliver the sample as part of a planned mass distribution.  Laboratories may also arrange to pick up the sample from the Caltrans Translab.

Testing and Reporting

Each RSP sample will include testing instructions and a results submittal form in paper format (hard copy). Electronic copies of the instructions (pdf format) and data submittal form (MS Excel format) will also be distributed by email. The instructions will include the test result reporting deadline, version of the test method to be used for testing and any other key information required for proper processing.

General testing and reporting requirements:

• Equipment used must have a documented current calibration
• A certified technician must perform the test
• The testing technician’s name must be reported in the test results

Failure to report the reference sample results by the deadline may result in suspension of the laboratory’s accreditation of the test method selected for a given round of proficiency testing.

RSP Results Evaluation

Caltrans IAP uses a statistical evaluation system to give each reference sample result a numerical rating.  This evaluation method uses the mean (X) and standard deviation (σ) for the entire RSP data set and compares it to the test result submitted by individual participating laboratories.  A laboratory’s result is compared to the overall average in terms of the number of standard deviations from the average (statistical value), and based on this comparison, is given the respective numerical rating.  For example, a laboratory that produces a result that is within 2.0 standard deviations (σ) of the overall RSP reporting average, but is not within 1.5σ, will be given a numerical rating of 3. 

Rating system for reference sample test results:

Unacceptable Results and Retesting

• Rating less than 3.0 on initial testing:  The laboratory must examine its equipment and/or test procedures to determine why the test results varied appreciably from the mean.  Caltrans IAP will ship a second RSP sample for retesting (first retest).  The retest results must be returned to the RSP coordinator within 15 calendar days of receipt of the retest sample.  If the laboratory produces an acceptable result, the RSP testing will be considered complete.
• Rating less than 3.0 on first retest:  The laboratory must submit a corrective action plan (CAP) to Caltrans IAP and another sample will be delivered for the witness test (second retest). The second retest must be scheduled with and witnessed by IA staff.  If the laboratory produces an acceptable result, the RSP testing will be considered complete.
• Rating less than 3.0 on second retest:  The laboratory’s accreditation will be suspended for the respective test method. Please see Section 2.5.2.3. of the Caltrans IA Manual, Appeal Procedure for a Laboratory, for steps required to lift the Laboratory Accreditation suspension.

Rating Letter and Final Report

Caltrans IAP will issue an RSP rating letter to the laboratory that includes the rating and overall performance.  The laboratory must keep a copy of the letter in its Caltrans Laboratory Accreditation Manual (CLAM).

Upon completion of all tests/retests by participating laboratories, the Reference Sample Program will post a final report on the Caltrans RSP website. Participating laboratories may request a copy of the final report by sending an email to Caltrans.Reference.Sample.Program@dot.ca.gov.

Final Reports:

• 2023 RSP CT 202
• 2023 RSP AASHTO T 209

Maintaining Laboratory Accreditation

A Caltrans-accredited laboratory is responsible for maintaining active status by participating in an RSP for a test method in which it holds accreditation.  Additionally, the laboratory must notify the RSP by email (Caltrans.Reference.Sample.Program@dot.ca.gov) of any changes in laboratory status such as laboratory manager contacts, telephone numbers, mailing address, email addresses, or scope of tests performed.  To update laboratory information, submit a TL-0119 RSP Questionnaire form to the RSP coordinator.

Repeated occurrences of non-participation or poor results may result in loss of accreditation of the test method selected for a given round of proficiency testing. 

Interlaboratory Studies (ILS)

Notable studies conducted by the ILS team:

• Development of CT 306 – Method of Test for Reducing Samples of Asphalt Mixtures to Testing Size
• Development of Precision Statement for AASHTO Test Method T 283 Using Rubberized Hot Mix Asphalt – Gap Graded for Asphalt Pavement
• Validation of California Test Method 389 Hamburg Wheel Track Test Using Rubberized Hot Mix Asphalt – Gap Graded for Asphalt Pavement
• Bulk Specify Gravity: Comparative Study for AASHTO T 166 and AASHTO T 275 for HMA Specimens Compacted under AASHTO T 312

In addition to developing and improving test methods, the ILS team also publish its studies to national and international publications and conferences. Notable papers published by the ILS team:

• Development and Verification of Sample Reduction Method for Rubberized Asphalt Mixtures in the Transportation Research Record (TRR)
• Development of Precision Statement for AASHTO Test Method T 283 Using Rubberized Hot Mix Asphalt – Gap Graded for Asphalt Pavement in the 2022 American Society of Civil Engineers (ASCE) International Conference on Transportation and Development
• Validation of California Test Method 389 Hamburg Wheel Track Test Using Rubberized Hot Mix Asphalt – Gap Graded for Asphalt Pavement in the 2022 American Society of Civil Engineers (ASCE) International Conference on Transportation and Development