The RPD myth...

We heartily welcome Phil Williams’ recent tutorial in NIR news, in which he offered a historically backed overview of the motivation and reasoning behind the concept of RPD (by the very originator himself). This was timely and no doubt appears as most useful for the young(er) generations in applied NIR spectroscopy. But this would hardly be the Mythbusters column were we not to have a serious issue to discuss. In short, Phil’s magisterial attempt does not clear up the key issues involved but rather compounds some of the reasons why the RPD index is perhaps not so well suited for its intended purpose after all. We will address the RPD concept by commenting on key statements made in the tutorial and will elucidate what’s really at the bottom of all the many different understandings of RPD and will later suggest a complementary index, perhaps even an alternative to RPD. We are well aware that we are about to tread on sacred ground here—for which reason we want to state at the beginning our undivided respect and high acclaim for the innumerable and invaluable achievements for the NIR community in the full oeuvre of Phil Williams. What follows therefore must be viewed as a strict scientific discussion, nothing more, nothing less—it is only the scientific arguments that count