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APPENDIX 3 - Background on statistical methods for bone densitometry reporting [ Appendix 3 Table of Contents ] ACCURACY and PRECISION Accuracy is defined as the ability of a technique to measure the true value of a physical property. For bone densitometry techniques it is usually assessed in vitro by measuring phantoms of known density, or in situ on cadavers; in the latter case the measured value is compared to a chemical analysis (ash weight) of the scanned bone. Accuracy is affected by systematic errors in the measuring technique caused by, for example, machine calibration, bad positioning, analysis errors and machine algorithms for handling intra- and extra-osseous fat. Systematic errors cause the measured value to be offset by a set amount from the true value; this amount of offset is constant in repeat measurements made under the same conditions. Accuracy is less crucial than precision in bone mineral measurements since it does not affect conclusions drawn regarding changes in BMD over time measured on the same machine. However, it is important when comparing density values measured using different techniques, in comparing density values obtained on bone densitometry machines from different manufacturers and in determining an initial diagnostic classification. Precision is related to the spread of results obtained when a measurement is repeated under exactly the same conditions. That is, it is a measure of the repeatability of the measurement. Precision is affected by random errors in the measuring technique. These are caused by fluctuations in the operating characteristics of the machine over time and operator errors in positioning patients and analysing results. Random errors cause the measured value to vary about a given mean when repeat measurements are made. Precision is probably the most important factor in routine bone densitometry measurements. Better precision allows smaller changes in bone density to be detected at any selected confidence level, and hence allows any change in bone status to be detected sooner (vide infra). Ideally, measurements should be both precise and accurate. The relationship between precision and accuracy is illustrated graphically in Figure 1.
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US Content copyright 2005 © Australian & New Zealand Bone & Mineral Society Design copyright 2005 © Anne Kovach, eRadiate |
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-- This page last edited: 13 Nov 2002 -- |
