Laboratory performance assessment based on MEAN +2 SD
| MEAN +2 SD | Original classification (before 1977) | Current classification (1977–99) |
|---|---|---|
| The scoring nomenclature (+2, +1, 0, −1) was introduced to harmonise the antibiotic assay scheme with the general microbiology schemes. | ||
| MEAN, the modulus of mean %error; SD, standard deviation around the mean %error. | ||
| 0–30 | Good | Acceptable, groups 1 and 2 |
| Score +2 | ||
| 30–50 | Poor | Borderline, groups 3 and 4 |
| Score +1 | ||
| 50–100 | Highly misleading | Poor, groups 5–9 |
| Score 0 | ||
| >100 | Highly misleading | Very poor, groups 10 and 11 |
| Score −1 | ||