The question
A potency assay fails its intermediate precision criterion of 8% CV. The proposed fix is retraining the analysts. Will that work?
The analysis
The study ran every combination of 3 analysts, 6 days, and 2 instruments in triplicate (108 results). A random-effects model splits the total variance into its sources, so each one can be sized separately.
| Source | CV contribution | Share of variance |
|---|---|---|
| Day to day | 2.5% | 53% |
| Instrument | 0.8% | 6% |
| Analyst | 0.9% | 7% |
| Repeatability | 2.0% | 35% |
| Repeatability (within run) | 2.0% | |
| Intermediate precision (all sources) | 3.4% |
Why it matters. Analysts account for a small share of the variability. Day-to-day effects dominate: something that changes each day, such as reagent or standard preparation, plate or cell condition, or instrument warm-up, moves the whole run. Retraining analysts would cost time and leave the failure in place.
What to do
- Investigate what changes between days: fresh reference standard and reagent preparation, cell passage or density, and system suitability results by run.
- Consider controls that absorb day effects, such as reporting relative to a same-plate reference or tightening run acceptance criteria.
- Repeat the variance components study after the fix, and set the criterion from the method's intended use, not from the data.
How it was done
Synthetic relative potency results from a fully crossed precision design. Variance components estimated by REML in a mixed model with analyst, day, and instrument as random effects; CVs expressed against the overall mean, consistent with ICH Q2 intermediate precision. Download the data (CSV).