How do I calculate the 90th percentile value to report the results of my lead and copper distribution samples?
The following instructions (taken from Chapter 246-290-025 WAC, 40 CFR Section 141.80) are written using lead as the example, but they apply in the same way to copper results. 1. Put all of your lead results in increasing order, starting with the smallest value and ending with the largest value, including non-detects. You’ll probably have several non-detects listed first, followed by a few detections. Make sure your detections are in increasing order. 2. Multiply the number of samples you took by 0.9. Round this result up to the next highest whole number. 3. Go to your ordered list of lead sample results and identify the sample that corresponds to the whole number from step 2 above. 4. The concentration of that sample is the 90th percentile. Here is an example: 1. 32 samples were collected. 2. 32 x 0.9 = 28.8. Round to 29. 3. Identify the 29th sample in your list of ordered samples (ordered from lowest to highest). 4. The concentration of lead for the 29th sample is the 90th percentile
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