Precision versus Accuracy (1)

1. Precision versus Accuracy (1) • Precision is the variation of X around <X> • expressed as standard deviation or variance • Accuracy is the closeness of <X> to the “true” value of X • Precision and accuracy are often mixed in the literature

2. Precision versus Accuracy (2) Precise, not accurate Accurate, not precise Precise and accurate Not accurate and not precise

3. Precision and true variance Precisionunderestimates true variance Precision equals true variance Precisionoverestimates true variance

4. Normal distributions and Z-scores X - average Z = -------- sigma

5. Normal distributions andRMS Z-scores X - average Z = -------- sigma RMS Z-score~0.5 RMS Z-score=1.0 (reference) RMS Z-score~2

6. Z-scores and RMS Z-scores • Local geometry RMS Z-scores: • Too tight restraining of geometry  0 < RMS Z-score < 1 • Proper Gaussian distribution  RMS Z-score 1 • Too loose restraining of geometry  1 > RMS Z-score • Structure Z-scores: • Z-scores > 0 are “better” than average • Z-scores < 0 are “worse” than average • However: AZ-score of -1 is equally likely as a Z-score of +1!!