Every measurement carries uncertainty from weighing, extraction, calibration, instrument response, the analyst and, above all, sampling. Laboratories accredited to ISO/IEC 17025 must estimate it. It is commonly reported as an expanded uncertainty (coverage factor k = 2, roughly 95% confidence), for example "total THC 21.0% ± 1.5% w/w".
In practice
That ±1.5% means a reading of 21.0% is consistent with a true value anywhere from about 19.5% to 22.5% for that sample. Two COAs of 20.1% and 21.7% from the same batch may not disagree at all. Uncertainty usually grows with heterogeneous samples: a single bud from the top of a plant can differ by several points from a lower one, which the lab's own uncertainty doesn't capture.
Same units as the result; also expressed as relative %. Expanded uncertainty U = k × combined standard uncertainty, with k = 2 for about 95% coverage.
Don't confuse with
- LOD/LOQ: thresholds at the low end, not the spread of a result.
- Lab-to-lab bias: a systematic offset, which uncertainty estimates may not include.
Myth check
Uncertainty explains small differences, not 10-point gaps. See buyer beware.
Homogenisation is grinding and mixing a sample so every portion the lab tests represents the whole; together with representative sampling it often matters more than the instrument.ISO/IEC 17025 is the international standard for the competence of testing and calibration laboratories; accreditation to it shows a lab's specific methods have been independently assessed.THC (Δ9-tetrahydrocannabinol) is the neutral, decarboxylated cannabinoid mainly responsible for cannabis's reported intoxicating effects; the living plant makes it almost entirely as THCA.The LOD (limit of detection) is the lowest amount of a compound a method can reliably tell apart from zero, but not necessarily measure accurately.The LOQ (limit of quantitation) is the lowest concentration a method can measure with acceptable accuracy and precision; below it a result is reported as <LOQ.Potency testing is laboratory measurement of a sample's cannabinoid content, normally by HPLC, reported as % w/w or mg/g for each cannabinoid plus calculated totals.Proficiency testing sends identical blind samples to many labs and compares their results with an assigned value, checking each lab's accuracy against its peers.Label inflation is the gap between the potency printed on cannabis packaging and what independent testing finds, a documented, mostly upward bias in retail THC claims.
