Relative Percent Difference or "RPD" is a measure of precision between two testing devices. Radon measurements usually do not produce exactly the same results, even when conducted simultaneously.
RPD = Difference (between two results) divided by the Average (of the same two results),multiplied by 100 = RPD
The EPA has guidelines established for interpreting results from radon tests conducted simultaneously. There is also an EPA requirement that at least one of the radon tests performed during a real estate screening measurement be a "dual device" test. The dual device strategy is yet another measure of precision error of the measurement method. Test kits are placed side by side (4" apart) in the area to be tested, at least 20" off the floor, 1 foot or more away from exterior walls, & 3 feet away from exterior doors, windows & draft sources. At the completion of the test, both devices are analyzed & the individual results as well as the average of the results are supplied. A condensed interpretation of the guidelines are as follows;
Both results < 4.0 pCi/l...provide results...if RPD is > 67%...investigate source of error...no mitigation required at this time.
One result > 4.0 pCi/l, one result > 4.0 pCi/l...if higher result is > twice lower result, the difference is greater than a factor of two & a retest should be conducted.
Both Results are > 4.0 pCi/l...provide results...if RPD is > 36%, investigate source of error...but since both results are > 4.0 pCi/l...either perform a confirmatory measurement or mitigate.
As listing agents, if one of your properties is being tested for radon by an inspector; be sure that at least on of the areas being tested has two devices placed next to one another. A single canister is not sufficient & may lead to unnecessary remedial action...OR - even worse - a problem gone undetected. The exception to this would be in cases where a continuous monitor is deployed...or the testing company plans on doing sequential testing (two consecutive test periods with both results submitted as one average). In my 23 + years of experience in the field, sequential testing is rarely - if ever employed.
As Realtors, You don't need to lose sleep over knowing these parameters. My purpose here is to let you know that there are guidelines & procedures such as checking Relative Percent Difference to help ensure the best possible approach to assessing whether or not a radon problem exists.
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