  This paper analyses the degree to which dialect classifiers based on
syntactic representations remain stable over space and time. While previous
work has shown that the combination of grammar induction and geospatial text
classification produces robust dialect models, we do not know what influence
both changing grammars and changing populations have on dialect models. This
paper constructs a test set for 12 dialects of English that spans three years
at monthly intervals with a fixed spatial distribution across 1,120 cities.
Syntactic representations are formulated within the usage-based Construction
Grammar paradigm (CxG). The decay rate of classification performance for each
dialect over time allows us to identify regions undergoing syntactic change.
And the distribution of classification accuracy within dialect regions allows
us to identify the degree to which the grammar of a dialect is internally
heterogeneous. The main contribution of this paper is to show that a rigorous
evaluation of dialect classification models can be used to find both variation
over space and change over time.
