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Release Date: June 8, 2010

Guideline No. CWP-10-001



## Composite Wood Products Airborne Toxic Control Measure (ATCM)

Title 17, California Code of Regulations, Sections 93120-93120.12



## Third Party Certification Guideline: Establishing a Correlation With an Acceptable Corre lation

Coefficient (“r”, Value)

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Establishing a correlation with an acceptable “r” v alue. Some third party certifiers (TPC) find it challenging to establish proper corre lations with five data pairs.

Section 93120.12, Appendix 2, subsection (g)(2) rea ds:

“Each plant’s quality control test results must be shown to correlate to primary or secondary method test results. The correlation mus t be based on a minimum sample size of five data pairs. If data shows variation f rom the previously used correlation, the

manufacturer shall work with the certifier to evalu ate the data to determine if a statistically significant change has occurred. If a change is noted, a new correlation curve will be established for the manufacturer by t he certifier.”

Program Issue : Concerning the number of data pairs that must be used to develop a correlation (i.e., a minimum of five), it is possib le that this number may not be sufficient for all Phase 2 compliant products because all of t he measurements are near-zero, or below the analytic method of detection. Unless a b road enough range in measured formaldehyde emissions for both the primary/seconda ry and small-scale test methods can be achieved (i.e., when the five data pairs for a product are tightly clustered), the correlation that is developed for the product is li kely to have a low “r” value. Guidance is needed with respect to the approaches that can be u sed to develop stronger correlations for products with clustered data.

Recommended CARB Guidance : In cases where data clustering occurs, it is recommended that a suitable correlation may be deve loped by:

• Collecting additional data to broaden the range in measured test data;

• Calculating a linear relationship using a point me asured near the origin (such as a concentration measured in an empty chamber) and t he average value of the clustered data points; or

• By establishing a small-scale test value threshold , in which the average smallscale test value for the data cluster is used as th e Quality Control Limit (QCL) for the product.For PB and MDF, TPCs are advised to develop a mill’ s correlation utilizing a minimum

formaldehyde emission test result range of 0.05 ppm (e.g., 0.03 to 0.08 ppm). Whenever possible, the measured emission test resul t range should be achieved by using only the product for which the correlation ap plies – it is not recommended that products other than the CARB-certified product manu factured by the mill be used to develop a mill’s correlation. It is further recomm ended that TPCs attempt to achieve a minimum acceptable “r” value for the correlation, i n consideration of the values in the following table:

Minimum “r” values for Certified Mill Product Corre lations 1

Degrees of Freedom (n-2) 2 “r” Value

3 0.878

4 0.811

5 0.754

6 0.707

7 0.666

8 0.632

9 0.602

10 or more 0.576

(1) From : The British Standards Institution. 2000. Wood-b ased panels. Sampling, cutting and inspection. Quality control in the factory. BS EN 326-2. 36 pp.

(2) “n” = number of primary/secondary and small-scale test data pairs. Correlations based on five data pairs have 3-degree s of freedom and must achieve an “r” value of 0.878 or greater.

Alternately, TPCs may also consider fitting a line through a point near the origin, to develop a two-point correlation, using the average primary/secondary and small-scale test value of the clustered data points for the pro duct as the second point. In this approach, the point near the origin may be a data p air representing the test value achieved when no sample is put in the primary/secon dary and small-scale test chamber. The second data point would be the average primary/ secondary and small-scale test result based on the paired data that has been colle cted (i.e., minimum of five). The slope of the line generated by this method would be used to determine the small-scale test result that represents the value equal to the Phase 2 emission standard using a primary/secondary method test.

A third option is to use a threshold approach in wh ich the average small-scale test value, calculated as above from a data cluster, is used as the Quality Control Limit (QCL) for the product. This value would be a “do not exceed” limit for the product that provides a margin of safety relative to the maximum value in t he data cluster.

For Phase 2 hardwood plywood (or other very low emi tting products), TPCs may need to collect more than five data pairs to achieve a corr elation with a minimum acceptable “r” value based on the above table or utilize the two-p oint correlation or threshold approach described above, given that the spread in test valu es of compliant products is likely to cluster in the 0.02 to 0.04 ppm range. In cases wh ere the major portion of the smallscale test values for a product are “non-detectable ” because the amount offormaldehyde collected is below the method limit of detection, consideration should be

given to using analytical methods with lower detect ion limits, so that fewer “zero” values are used to calculate the average test value of the data cluster. Lower limits of detection can be achieved by using reagents other than chromo tropic acid (e.g., 3-methyl-2- benzothiazolone or acetyl acetone). Regarding the use of ASTM D 5014, additional information concerning 3-methyl-2-benzothiazolone c an found in Chan et al. (2001) in The Analyst, Volume 126, pages 720-723. Detection limits can also be lowered for some test methods by lowering background formaldehy de levels.