Duplicate Precision Failure in a Multifamily Sample
A 24-unit, three-story apartment building on a slab-on-grade foundation has no basement. The owner orders a multifamily radon survey before refinancing. Following the applicable multifamily protocol, the measurement professional selects three ground-floor units, a 10% initial sample. In unit 12, an end unit with a sliding glass door, two side-by-side continuous monitors are deployed for 48 hours. The results are 7.4 pCi/L and 2.6 pCi/L. The higher monitor sat on a bookshelf 18 inches from the sliding door; the lower monitor was on an interior wall 3 feet away. The higher monitor's data show relative humidity rising 20 percentage points while the other monitor's humidity is stable, and the higher unit's graph shows a sharp drop near the end of the test. Units 4 and 9 read 1.2 and 1.8 pCi/L. The property manager wants to use the 2.6 result, avoid tenant notification, and close the survey this week. The professional must evaluate the duplicate pair and the need for expanded testing.
1. Evaluate the duplicate pair. Is the 7.4 pCi/L result usable, and what explains the discrepancy?
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The pair fails a basic precision check. The relative percent difference is about 96%: the absolute difference of 4.8 divided by the average of 5.0, times 100. That is far beyond the usual duplicate acceptance criterion of about 36% for results at or above 4 pCi/L. One or both monitors are suspect. The high monitor's placement is problematic: 18 inches from a sliding door is close to an exterior opening where door use and outdoor air can affect the reading. Its humidity rise of 20 points while the other monitor stays stable suggests different air exposure or condensation, and the sharp drop near the end suggests an event affecting that monitor. Do not simply keep the low number. Review both data logs, calibration, and placement. If the high monitor was improperly placed, its result may be invalid, but that does not validate 2.6 by default; it leaves only one acceptable monitor in the unit. Retest unit 12 with properly placed duplicates before drawing a conclusion.
2. Does the initial 10% sample satisfy the survey, and what should happen next given these results?
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The three-unit sample is only an initial screening sample; it does not become a final clearance because one duplicate is inconsistent. A failed duplicate means unit 12 has not produced a reliable result. The other units at 1.2 and 1.8 pCi/L do not compensate for that gap. Retest unit 12 under closed-house conditions with two properly placed monitors away from the sliding door and exterior wall. If the retest confirms an elevated result, the applicable multifamily protocol requires expansion beyond the initial sample, commonly to additional ground-contact and adjacent units, and the owner must be notified. If the retest is low and the duplicate precision is acceptable, document why the original pair was rejected. In either case, the survey cannot be closed this week on the current data.
3. What should you tell the property manager who wants to avoid notification and use the 2.6 result?
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Tell the manager that you cannot select the lower result to avoid notification. The duplicate pair fails precision, so neither number is a confirmed unit result. Reporting 2.6 as the unit result would misrepresent an unverified measurement and could delay mitigation if the unit is actually elevated. Explain the sequence: invalidate the pair, retest unit 12, and expand testing if an elevated result is confirmed. Notification obligations depend on the applicable protocol and state or local requirements, but they are not yours to waive because a refinancing deadline is inconvenient. Put the refusal and the reason in writing. If the manager insists, do not alter the report; escalate the conflict and preserve the original records. The professional judgment is to protect the occupants and the integrity of the survey, not the transaction timeline.