TREATMENT OF VALUES BELOW THE DETECTION LIMITS IN CORRELATION-ANALYSIS OF CHLORINATED DIOXINS AND RELATED-COMPOUNDS

Concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) in incinerator exhaust gas were correlated with concentration s of other aromatic compounds. Values below the limits of detection (L OD) are treated as missing. However, by doing so misleading results co ncerning whether or not a correlation exists may be produced. To addit ionally verify the results, we propose the following procedure. Replac e non detected concentrations by any value smaller than the minimum of detected concentrations and compute Spearman's rank correlation coeff icient. If this coefficient is not significant it is proposed to assum e that no correlation exists.