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FIGURE 7 Mean pme vs mean bit rate using the personal gold standard. The dotted, dashed, and dash-dot curves are quadratic splines fit to the data points for judges 1, 2, and 3, respectively. The solid curve is a quadratic spline fit to the data points for all judges pooled.

mean bit rate per compression level (bpp)

mean bit rate per compression level (bpp)

Results Using the Personal Gold Standard

As was discussed previously, the personal gold standard was set by taking a radiologist's recorded vessel size on the uncompressed image to be the correct measurement for judging her or his performance on the compressed images. Using a personal gold standard in general accounts for a measurement bias attributed to an individual radiologist, thereby providing a more consistent result among the measurements of each judge at the different compression levels. The personal gold standard thus eliminates the interobserver variability present with the independent gold standard. However, it does not allow us to compare performance at compressed bit rates to performance at the original bit rates, since the standard is determined from the original bit rates, thereby giving the original images zero error. As before, we first consider visual trends and then quantify differences between levels by statistical tests.

Figure 7 shows average pme vs mean bit rate for the five compressed levels for each judge separately and for the judges pooled, whereas Fig. 8 is a display of the actual pme vs. actual achieved bit rate for all the data points. The data for the judges

FIGURE 8 Pme vs actual bit rate using the personal gold standard. The x's indicate data points for all images, pooled across judges and compression levels. The solid curve is a quadratic spline fit to the data.
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