Next, we compared the difference in PCE between the closest vectors (Fig. ). When we calculated the nearest neighbor distance using the best representation, 90% of the data were within the distance 5. It was also found that the difference in PCE was distributed more widely for the closer vectors, ranging from -20% to 30%. In many cases, the distance was zero, i.e., identical vectors were created by degenerating from different data corresponding to different PCEs. This data degeneracy negatively affects the regression performance.