Number of citations of the different thematic categories of the ISO standards and VDI guidelines sorted by years. The ISO standards 18458 and 18459 were published in 2015 and ISO 18457 in 2016. Earlier citations refer to VDI guidelines published from 2012 onwards. The numbers of citations (n) of the respective categories are as follows: terminology, n = 36; approach, n = 14; application, n = 8; evaluation, n = 5; and state of the art, n = 7