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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 Correlation 

Coefficient (“r”, Value) 

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Establishing a correlation with an acceptable “r” value. Some third party certifiers 

(TPC) find it challenging to establish proper correlations with five data pairs.

Section 93120.12, Appendix 2, subsection (g)(2) reads: 

“Each plant’s quality control test results must be shown to correlate to primary or 

secondary method test results. The correlation must be based on a minimum sample 

size of five data pairs. If data shows variation from the previously used correlation, the 

manufacturer shall work with the certifier to evaluate 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 the 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 possible that this number may not be sufficient 

for all Phase 2 compliant products because all of the measurements are near-zero, or 

below the analytic method of detection. Unless a broad enough range in measured 

formaldehyde emissions for both the primary/secondary 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 likely to have a low “r” value. Guidance is 

needed with respect to the approaches that can be used 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 developed by: 

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

• Calculating a linear relationship using a point measured near the origin (such as 

a concentration measured in an empty chamber) and the 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 the 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 result range should be achieved by 

using only the product for which the correlation applies – it is not recommended that 

products other than the CARB-certified product manufactured by the mill be used to 

develop a mill’s correlation. It is further recommended that TPCs attempt to achieve a 

minimum acceptable “r” value for the correlation, in consideration of the values in the 

following table: 

Minimum “r” values for Certified Mill Product Correlations1

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-based 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-degrees 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 product as the second point. In this 

approach, the point near the origin may be a data pair representing the test value 

achieved when no sample is put in the primary/secondary 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 collected (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 which 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 the data cluster. 

For Phase 2 hardwood plywood (or other very low emitting products), TPCs may need to 

collect more than five data pairs to achieve a correlation with a minimum acceptable “r” 

value based on the above table or utilize the two-point correlation or threshold approach 

described above, given that the spread in test values of compliant products is likely to 

cluster in the 0.02 to 0.04 ppm range. In cases where the major portion of the smallscale test values for a product are “non-detectable” because the amount of 
formaldehyde collected is below the method limit of detection, consideration should be 

given to using analytical methods with lower detection 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 chromotropic acid (e.g., 3-methyl-2-

benzothiazolone or acetyl acetone). Regarding the use of ASTM D 5014, additional 

information concerning 3-methyl-2-benzothiazolone can 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 formaldehyde levels.