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

Precision versus Accuracy (2)

Precise,not accurate

Preciseand accurate

Accurate,not precise

Not accurateand not precise

Precision and true variance

Precision underestimatestrue variance

Precision equals

true variance

Precision overestimatestrue variance

Normal distributions and Z-scores

X - average Z = -------- sigma

Normal distributions andRMS Z-scores

RMS Z-score=1.0 (reference)

RMS Z-score~2

RMS Z-score~0.5

RMS Z-score= sum Z 2 n

X - average Z = -------- sigma

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: A Z-score of -1 is equally likely as a Z-score of

+1!!