If I tell you that the APC spending has increased, do you immediately realize that there can only be two explanations (the volume of articles has increased and/or the price level has increased)?
@fBedecarrats Yes indeed, these contribute to the increase in the volume of articles with APC.
@enroweb can't that come also from an increase of the share of journals requesting APCs or from an increase in the share of authors wanting to have open access instead of gated publications?