Studies in the field of sociolinguistics typically take a sample population and interview them, assessing the realisation of certain sociolinguistic variables. Labov specifies the ideal sociolinguistic variable to
be high in frequency,
have a certain immunity from conscious suppression,
be an integral part of larger structures, and
be easily quantified on a linear scale.[5]
Phonetic variables tend to meet these criteria and are often used, as are morphosyntactic variables, morphophonological variables, and, more rarely, lexical variables. Examples for phonetic variables are: the frequency of the glottal stop, the height or backness of a vowel or the realisation of word-endings. An example of a morphosyntactic variable is the frequency of negative concord (known colloquially as a double negative). Two well-known and frequently studied morphophonological variables are T/D deletion, the optional deletion of the sound /t/ or /d/ at the end of a word, as in "I kep' walking" (Wolfram 1969;[6] Labov et al. 1968[7]); and the ING variable, the optional pronunciation of -ing at the end of a word as -in', as in "I kept walkin'" (e.g. Fisher 1958;[8] Labov 1966/1982;[9] Trudgill 1974[10]).