Norland Elite 437A150, Operator'S Manual

Page: 127 / 490

Operator's Guide
437D140 Rev. I
Basic Operation 4-19
the serial number of the QC Phantom used at the facility
the assigned value of BMD, Fat, and Lean values used to plot the Accuracy graph (Fat and Lean only
entered when Soft Tissue Calibration is enabled)
and the Standard Deviation of each BMD, Fat, and Lean value used to plot the Precision graph.
The assigned values were entered into the system at installation, based on factory characterization of the sys-
tem’s QC Phantom.
QC RESULTS AREA: shows the date of the last Calibration Scan and its resulting BMD and BMC values. If
Soft Tissue Calibration is enabled (an option), the Fat and Lean results are also shown.
Below the last scan date is the numeric result of the last 16 QC’s. There, the mean value and C.V. for the most
recent 16 scans is displayed. If fewer than 16 scans have been performed, all calculated values are used in the
computation.
Next, there are two graphs based on the QC Phantom scans.
UPPER GRAPH: displays the precision of the Norland system. The values from the last 180 days (6 months)
of QC Phantom scans are displayed, showing their scatter about the mean. The two dashed horizontal lines
indicate ±2 S.D. about the mean. The points shown between the dotted lines are within ±2 standard deviations.
Statistically, 95% of the values obtained will fall in this range. Note that 5% of the points, or 1 out of 20, are stat-
istically expected to lie outside the dashed lines. Points shown outside the dashed lines are those scans that
exceed the ± 2 S.D.
LOWER GRAPH: displays the accuracy of the system, based on QC Phantom scans. The mean values of the
last 180 days of calculations (6 months) are plotted against the assigned BMD value (or Fat, or Lean values) of
the QC Phantom. The two dashed horizontal lines indicate deviations of ± 1.5% from the BMD assigned value
(or ± 2% from the Fat and Lean values).
QC Results Statistics Messages
The tables that follow describe the possible message readouts for the Precision and Accuracy results. They are
found in the
QC Results area of the QA Results tab window (see Sample QA Results tab window ).
The criteria for generating the Precision and Accuracy warning messages are modeled after those of Shewart
(see Table 2 in Orwoll, et. al., J. Bone and Mineral Res. 6:1991, p.196.)
PRECISION
MESSAGE
MESSAGE
INDICATION
OPERATOR
ACTION
OK Precision of the system is within specification. No Action Required
OUT OF RANGE Standard deviation estimated from 16 scans is greater than the
expected value. Repeat QA ("Calibrate the System Using the
QA Calibration Standard" on page 4-8)
WARNING 1 A single value is outside ± 3 standard deviations from the mean. Repeat QA ("Calibrate the System Using the
QA Calibration Standard" on page 4-8)
WARNING 2 Two consecutive values exceed ± 2 standard deviations from
the mean. Repeat QA ("Calibrate the System Using the
QA Calibration Standard" on page 4-8)
WARNING 3 Four consecutive values of the same sign (either plus or
minus), exceed one standard deviation from the mean. Repeat QA ("Calibrate the System Using the
QA Calibration Standard" on page 4-8)
Table 4-8: QA Results tab Precision Messages and Actions