There were several sources available for the description of number marking in MORPHOLOGY 4.1.1. Firstly, the studies of Harder, Koolhof & Schermer (2003) and Zwitserlood & Nijhof (1999) provided a solid basis. Harder, Koolhof & Schermer used two datasets; the first one consisted of signed texts on two CD-ROMS that functioned as dictionaries, and texts on old video tapes. The CD-ROM texts were example texts to illustrate how the signs could be used, and were between one and five sentences long. The videotaped texts were part of NGT acquisition material and functioned as homework exercises. Both types of texts had been recorded for the purpose of documentation and teaching and are, thus, not natural or spontaneous; yet, given that they had also not been recorded with the specific aim of investigating plural marking, they were still informative. These texts yielded 291 plural forms in their first dataset. Secondly, they discussed over 600 nouns from another dictionary CD-ROM to check if and how the plural form is marked on these nouns. The researchers do not mention specifically with whom they discussed the signs, but they mention that the research was done by a deaf teacher of NGT and a hearing researcher. They investigated several ways of plural marking, e.g. also verbal marking, and restrictions on nominal reduplication.
Zwitserlood & Nijhof performed an elicitation task in which the participants were asked to describe simple pictures, with entities in singular or in plural form represented. The participants were four native signers, of various ages: the youngest was 23, the oldest 50 years old. Two of them came from Utrecht and two from Amsterdam. The researchers used 68 pictures, of which 26 showed singular objects, 22 plural objects arranged neatly, and 20 objects arranged randomly. The latter two types of pictures were included to also look into distributional effects, and these results were, thus, also useful for MORPHOLOGY 4.2.
Secondly, I was lucky to have a preliminary data set available on reduplication in the Corpus NGT, compiled by researcher Cindy van Boven, who, at the time, worked on her PhD project on reduplication in NGT. Van Boven used the Corpus NGT, and looked into phonological properties of nouns and the type of marking that these nouns received.
Thirdly, since the literature studies are from 1999 and 2003, respectively, I checked with a deaf informant whether some of the conclusions made by the researchers were still valid. The deaf informant was a woman of around 60 years old, who lived in the South of the Netherlands and in the Amsterdam region. I had several discussion rounds with her, some in which I gave a context and then asked for the plural form of a sign X, some in which I provided pictures and asked her to describe these, some in which I produced a plural form myself and asked whether this was acceptable. These discussions were also the main source of information for the descriptions in Section 4.2, in which I also provided pictures of objects in different arrangements (neatly ordered or in random distribution).
As for the information on non-manual marking in MORPHOLOGY 4.1.2, Schermer (2001) is based on the PhD dissertation of Schermer (1990), in which she elicited data from six informants from Groningen and Amsterdam (21-45 years old). The data consists of signed translations of two Dutch written fairytales, signed stories derived from a picture-book, and spontaneous conversations. One of the fairytales was translated by all six participants, and one by five of them. Two informants participated in the retelling of the picture-book, and all participants were involved in spontaneous conversations. In every task, one participants functioned as the main signer, and another as audience.