  In recent work we have presented a formal framework for linguistic annotation
based on labeled acyclic digraphs. These `annotation graphs' offer a simple yet
powerful method for representing complex annotation structures incorporating
hierarchy and overlap. Here, we motivate and illustrate our approach using
discourse-level annotations of text and speech data drawn from the CALLHOME,
COCONUT, MUC-7, DAMSL and TRAINS annotation schemes. With the help of domain
specialists, we have constructed a hybrid multi-level annotation for a fragment
of the Boston University Radio Speech Corpus which includes the following
levels: segment, word, breath, ToBI, Tilt, Treebank, coreference and named
entity. We show how annotation graphs can represent hybrid multi-level
structures which derive from a diverse set of file formats. We also show how
the approach facilitates substantive comparison of multiple annotations of a
single signal based on different theoretical models. The discussion shows how
annotation graphs open the door to wide-ranging integration of tools, formats
and corpora.
