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Precision versus Accuracy (1) Precision is the variation of X around – expressed as standard deviation or variance Accuracy is the closeness of to the “true” value of X Precision and accuracy are often mixed in the literature
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Precision versus Accuracy (2) Precise, not accurate Precise and accurate Accurate, not precise Not accurate and not precise
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Precision and true variance Precision underestimates true variance Precision equals true variance Precision overestimates true variance
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Normal distributions and Z- scores X - average Z = -------- sigma
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Normal distributions and RMS Z-scores RMS Z-score=1.0 (reference) RMS Z-score~2 RMS Z-score~0.5 X - average Z = -------- sigma
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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!!
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