Norland Elite 437A150, Operator'S Manual

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Accuracy
Accuracy is the degree to which a measurement value reproduces the actual value of the quantity being meas-
ured.
Although the accuracy of a single measurement includes the imprecision of that measurement, it is customary
to specify instrumental accuracy with the imprecision effect removed. This is called “accuracy of the mean”
because taking the mean or average of many measurements minimizes the imprecision. The accuracy spe-
cifications for the Norland Bone Densitometer are stated in terms of accuracy of the mean.
Several manufacturers have adopted phantoms fabricated of calcium hydroxyapatite and epoxy. The cal-
ibration of each Norland instrument is based on hydroxyapatite phantoms constructed to specifications
described by White (White, D. R., Martin, R.J., and Darlinson, R.: “Epoxy resin based tissue substitutes.” Brit.
J. Radiol. 50, 814-821, 1977).
Standard Deviation (S.D.)
Standard Deviation is used to specify precision. In a normal (or bell-curve) distribution, 68% of all the estim-
ations lie within one standard deviation of the mean, and 95% of all measurements lie within two standard devi-
ations of the mean.
Coefficient of Variation (C.V.)
Coefficient of Variation is used to specify precision. It is the standard deviation expressed as a percent of the
measured quantity. The coefficient of variation is the standard deviation divided by the mean.
Reference Set Comparison
The patient's Bone Mineral Density (BMD) estimate can be compared to a reference population that is clinically
free of bone disease and matches the patient's sex, ethnic background, and age. This variance of the patient's
BMD value relative to the mean of the reference population is graphed and expressed as a standard deviation.
Norland software contains many reference sets for comparisons and facilities can create their own reference
sets based on local populations.
Reference Set upgrades are provided free of charge and are usually distributed as they become available.
Reference sets are displayed as a chart containing the average BMD value of the reference population for the
prescribed anatomy. Many reference charts are color coded to assist with Fracture Risk Assessment. A
sample chart is displayed in Figure 2-4.
The middle line represents the average value for BMD for the ages included in the chart. The dashed lines
above and below the middle line are (+2) and (-2) standard deviations from the mean value.
Operator's Guide
437D140 Rev. I
2-10 General Information