Reference change values and power functions
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Natàlia Iglesias Canadell
Abstract
Repeated samplings and measurements in the monitoring ofpatients to look for changes are common clinical problems. The “reference change value”, calculated as zP×[2×(CVI2 + CVA2)]1/2, where zP is the z-statistic and CVI and CVA are within-subject and analytical coefficients of variation, respectively, has been used to detect whether a measured difference between measurements is statistically significant. However, a reference change value only detects the probability of false-positives (type I error), and for this reason, a model to calculate the risk ofmissing significant changes in serial results from individuals (probability off alse-negatives) is investigated in this work by means of power functions. Therefore, when an analyte is being monitored in a patient, power functions estimate the probability of detecting a defined real change by measuring the difference. Thus, when a measured difference is the same as the calculated reference change value, then it will be detected in only 50% of situations.
Copyright © 2004 by Walter de Gruyter GmbH & Co. KG
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