Ellis et al [ Ellis CACM ] also define the two dimensions of ``common task'' and ``shared environment''. Increasing points along the ``common task'' dimensions represent increasing commonality in the tasks carried out by the collaborators and increasing points along the shared environment dimension represent increasing amounts of shared information. Ellis et al put time sharing systems low in the common task dimension, a software review system high in the common task dimension, electronic mail system low in the shared environment dimension, and finally, an electronic classroom high in the shared environment dimension.
It is difficult to precisely define these dimensions and the points along it, and so it is difficult to use them to classify applications. It may be possible to order applications along the shared environment spectrum -- an application that allows sharing of more kinds of objects is higher on the spectrum. The kinds of objects could include shared artifacts such as group drawings and documents, annotations and comments about them of the kind provided by Quilt and PREP, static images of the participants, dynamic videos of the participants, and notification mail messages of the kind provided by Mercury, Quilt, Coordinator, and Information Lens. However, it is difficult to precisely define the common task spectrum. One definition, which is consistent with Ellis et al's usage, can be based on synchrony -- an application that allows users to view and respond quicker to other user's input would be higher on the common task spectrum.
So one ordering of our example applications in the Shared Environment Spectrum may be: Coordinator < Quilt
since Coordinator supports only notification messages while Quilt also supports shared documents and annotations. Similarly, one ordering of our example application in the Common Task/Synchrony dimension may be: InformationLens < Ensemble < Grove
Ensemble sends changes after they are committed, and Grove sends changes incrementally. since InformationLens requires explicit sends and receives, A definition of common task based on synchrony is not entirely satisfying since it is possible for users of synchronous applications to not have much commonality in tasks - consider a user (e.g. manager) simply monitoring the activities of people in a shared awareness space or users of MUD. Perhaps another definition is that an application that provides more facilities to support conference discovery is low in the common task dimension. Yet another definition is to use the interdependencies among the activities of of different users as a measure of common task. We can use Malone's work described later to categorize interdependencies. The challenge is to identify tool features for supporting them, so that we can use the common task dimension to categorize tools.