Variation is a characteristic of language: there is more than one way of saying the same thing. Speakers may vary in pronunciation (accent), word choice (lexicon), or morphology and syntax (sometimes called "grammar"). But while the diversity of variation is great, there seem to be boundaries on variation – speakers do not generally make drastic alterations in word order or use novel sounds that are completely foreign to the language being spoken. Linguistic variation does not equate to language ungrammaticality, but speakers are still (often unconsciously) sensitive to what is and is not possible in their native lect.
Variationists study how a language changes by observing it. This is accomplished by looking at authentic data. For example, variation is studied by looking at linguistic and social environments, then the data is analyzed as the change occurs. Variation in research programs must be malleable due to the nature of language itself. This is because language is also fluid in transition and does not shift from one state to another instantaneously.
Language variation is a core concept in sociolinguistics. Sociolinguists investigate whether this linguistic variation can be attributed to differences in the social characteristics of the speakers using the language, but also investigate whether elements of the surrounding linguistic context promote or inhibit the usage of certain structures.
Studies of language variation and its correlation with sociological categories, such as William Labov's 1963 paper "The social motivation of a sound change," led to the foundation of sociolinguistics as a subfield of linguistics. Although contemporary sociolinguistics includes other topics, language variation and change remains an important issue at the heart of the field.
Sociolinguistic variables
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. 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; Labov et al. 1968); 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; Labov 1966/1982; Trudgill 1974).
Analysis and methodology
Analyzing sociolinguistic variation often involves the use of statistical programs to handle its multi-variable nature. One essential part of the methodology is to count up the number of tokens of a particular variant and compare it to the number of times the variant could have occurred. This is called the "Principle of Accountability" in Tagliamonte (2012). Comparing the tokens to the total number of words in a corpus or comparing one corpus to another leads to erroneous results. This count of the possible occurrences can be difficult at times because some variants alternate with zero (such as relative pronouns that, who, and zero).
In 1970 Eugenio Coșeriu, revisiting De Saussure's synchrony and diachrony distinction in the description of language, coined the terms diatopic, diastratic and diaphasic to describe linguistic variation.Eugenio Coșeriu (1970) Einführung in die strukturelle Betrachtung des Wortschatzes
Association with age
There are several different types of age-based variation one may see within a population. They are: vernacular of a subgroup with membership typically characterized by a specific age range, age-graded variation, and indications of linguistic change in progress.