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Niton XLp 300, 9/24/2004, ed. I If the difference is less than the Retest Tolerance Limit, the inspection has passed the retest. If the difference of the overall averages equals or exceeds the Retest Tolerance Limit, this procedure should be repeated with ten new testing combinations. If the difference of the overall averages is equal to or greater than the Retest Tolerance Limit a second time, then the inspection should be considered deficient. Use of this procedure is estimated to produce a spurious result approximately 1% of the time. That is, results of this procedure will call for further examination when no examination is warranted in approximately 1 out of 100 dwelling units tested. TESTING TIMES: For the Lead-in-Paint K+L variable reading time mode, the instrument continues to read until it is moved away from the testing surface, terminated by the user, or the instrument software indicates the reading is complete. The following table provides testing time information for this testing mode. The times have been adjusted for source decay, normalized to the initial source strengths as noted above. Source strength and type of substrate will affect actual testing times. At the time of testing, the instruments had source strengths of 26.6 and 36.6 mCi. Testing Times Using K+L Reading Mode(Seconds) All Data Median for laboratory-measured lead levels (mg/cm2) Substrate 25th Median 75th Pb<0.25 0.25<Pb<1.0 1.0<Pb Percentile Percentile Wood 4 11 19 11 15 11 Drywall Metal 4 12 18 9 12 14 Brick 8 16 22 15 18 16 Concrete Plaster CLASSIFICATION RESULTS: XRF results are classified as positive if they are greater than or equal to the threshold, and negative if they are less than the threshold. DOCUMENTATION: A document titled Methodology for XRF Performance Characteristic Sheets provides an explanation of the statistical methodology used to construct the data in the sheets, and provides empirical results from using the recommended inconclusive ranges or thresholds for specific XRF instruments. For a copy of this document call the National Lead Information Center Clearinghouse at 1-800-424-LEAD. This XRF Performance Characteristic Sheet was developed by the Midwest Research Institute (MRI) and QuanTech, Inc., under a contract between MRI and the XRF manufacturer. HUD has determined that the information provided here is acceptable when used as guidance in conjunction with Chapter 7, Lead-Based Paint Inspection, of HUD's Guidelines for the Evaluation and Control of Lead-Based Paint Hazards in Housing. 3 of 3