One cannot rule out that we have overlooked published cold gas phase vibrational work on a system in the test set, or that such work appears on the scientific record before the experimental HyDRA data are released after the end of the challenge. This could be due to activities of other experimental groups not involved in the challenge, or due to data leakage from our experimental groups despite substantial effort to keep the results confidential among the contributing experimentalists. In such a case, the affected test set member will be shifted to training set status, because one cannot rule out that some model-proposing participants had a knowledge advantage over others. The goal is clearly to keep the blind character as unquestionable as possible .