Our initial attempt at a predictive and normative account of grounding behavior is given in (1), where the left side represents the probability that a particular action which communicates will be performed.
GC represents the Grounding criterion for , if this is low, there is no need for the information to be grounded, and thus a low probability that an agent will perform some action to ground it. The importance for grounding a particular piece of information also depends on the cost (with respect to the task) of non-grounding - how will task performance degrade if the particular is not grounded. An important factor in this is the persistence of the particular - it is a waste of time to ground highly transient information that changes before it's current value can be of use.
G represents the prior groundedness of . If it is already well grounded, there is little need to perform further action, even if it is crucial that the information be grounded. G will depend in part on how much and what kind of information is provided automatically by the environment (e.g., the shared visual situation of the whiteboard). Another important factor is the perplexity of the information - the probability that some information believed to be grounded could be misunderstood or disagreed upon.
C represents the collaborative grounding cost of , including not only the effort required to perform the action, but also the affiliated costs of understanding it as conveying , including potential further actions (repairs) which may be required. If these costs are high, there is not as much utility in performing the action, while if they are low, the action may be performed, even when grounding is not particularly crucial. C depends not only on the features of the medium itself, but also on the matching between the subject and the medium, e.g., how familiar the subject is with the medium. For example sometimes collaborators use an apparently more expensive medium simply because they know how to use it and are reluctant to learn something new, which requires an extra learning cost.
All of these aspects must, of course, be put relative to the beliefs of a particular agent in order to be used predictively. Note, also, that any of them can change throughout the course of a collaboration. The importance that something be part of common ground is relative to the local circumstances as well as the overall goals. Agents can also change their minds as to whether something has been grounded or not. Third or fourth turn repair of misunderstanding [McRoy and Hirst1995] comes from precisely this case, where an agent previously believed some had a relatively high degree of groundedness, but later information reduced this level. Similarly, local context will play a large role in determining whether a particular will be effective at communicating some .
Further work is still needed to try to quantify the relative aspects of this relation with respect to each other. In order to adapt (0) from a proportionality to a true equality that could be used to calculate the probability (or utility) of action, it is also necessary to consider two other factors: what other actions might better convey , and what other actions (including both communicating some , as well as other non-communicative task-related acts) might also be useful to perform. For the former, the grounding criterion and prior groundedness of will still be useful, while for the latter some more global arbitration strategy must be used to set priorities. In some domains, time, focus, and local context are also important factors - sometimes one must perform some acts before the balance of costs and benefits have been changed, while other acts have more flexibility.