Due to the detailed nature and use of actual reserves of actual resources and their real-world price trends, the indexes have been interpreted as a prediction of the number of years until the world would "run out" of them, both by environmentalist groups calling for greater conservation and restrictions on use, and by skeptics criticizing the predictions accuracy. This interpretation has been widely propagated by media and environmental organizations, and authors who, apart from a note about the possibility of the future flows being "more complicated", did not clearly constraint or deny this interpretation. While environmental organizations used it to support their arguments, a number of economists used it to criticize LtG as a whole shortly after publication in 70's (Peter Passel, Marc Roberts, and Leonard Ross), with similar criticism reoccurring from Ronald Baily, George Goodman and others in 90's. In 2011 Ugo Bardi in "The Limits to Growth Revisited" argued that "nowhere in the book was it stated that the numbers were supposed to be read as predictions", nonetheless as they were the only tangible numbers referring to actual resources, they were promptly picked as such by both supporters as well as opponents.