According to IEEE Standard 1241-2010 for Terminology and Test Methods for Analog-to-Digital Converters you have a missing code if the following inequation is true (page 44):
\$ DNL[k] \leq -0.9 \$
But according to the book: Analog-to-Digital Conversion by Pelgrom(Second Edition) you have got a missing code when DNL = -1.
From my point of understanding the second definition should be the right one because once a step has got twice the size of a normal LSB, a code is missing.
So here is my question:
Which one of these two definitions is the right one or are both valid?