Box & Whisker graph of the absolute difference in the log χ² of the reconstructed to the normalised spectra for the 384 test spectra between the 9th SPC iteration and each of the previous iterations ( | log χ²(9) − log χ²(i) | ). The box is constrained by the 25th and 75th percentiles, and the red lines are the medians. The whiskers extend to the furthest data points not considered to be outliers.