Figure 2.

Results after executing procedures used to eliminate machine dependent variations for the 22 vendor‐protocols for the middle density foam (nominal density 192.6 kg/m³). Upper panel: trace plot for each of the 22 vendor‐protocols at each of the calibration steps. Lower panel: boxplot representation for each vendor‐protocol. The box represents the first and third quartile of the distribution, and the horizontal bar in the middle of the box is the median value. The upper and lower whiskers are the maximum and minimum values excluding outliers. The data points outside of the whiskers are considered outliers. The reference value (solid line) at 80 keV is calculated based on the SRM foam density and assumed composition, and the mean value (dashed line) is the unweighted mean of the HU value at 80 keV for the entire group. The maximum variation and standard deviation in HU at each step are also listed. All the data analysis and graphics were performed with R (http://www.R-project.org).