Quality Assurance and Quality Control
What you must understand
QA is the planned system; QC is the checks that verify it. Key QC elements include duplicates (precision), blanks (contamination), spikes (accuracy), calibrations, and control charts. Duplicate results should agree within a stated relative percent difference (RPD); a common criterion is ≤25% or within the device's stated precision. Blanks should read at or near zero - a high blank signals contamination or handling error. Spikes verify recovery. Devices are calibrated against a known radon source traceable to a national standard; the EPA Radon Measurement Proficiency (RMP) program and device-specific performance criteria apply. Chain of custody, deployment/retrieval logs, and documented deviations are essential. Measurement error budgets combine counting statistics, calibration uncertainty, and field variability. Outliers should be investigated, not discarded silently.
Traps
Questions swap duplicates (precision) with spikes (accuracy), or treat blanks as precision checks. Don't assume a single high duplicate is acceptable - investigate. Remember that QC failures require documentation and possible re-testing.
Worked example
Two duplicates read 4.0 and 5.2 pCi/L. RPD = |4.0−5.2| / ((4.0+5.2)/2) × 100 = 26%. Slightly above a 25% criterion - flag, investigate handling, and consider re-testing. Rule of thumb: duplicates = precision, spikes = accuracy, blanks = contamination.