What factors indicate a special study is more appropriate than a DSS?

What factors in a decision situation indicate that it 
is probably more appropriate to prepare a special 
study than it is to build a Decision Support System?

Let's clarify the term special study. Some other names
that are used include: a quantitative analysis, a simulation
study, and a management analysis. Special studies are usually
prepared to support decision-making in situations that 
are especially important and one-time (novel or infrequent).
Situations that are very unstructured, involve negotiation or
bargaining or that are political are also likely candidates
for special studies. DSS are not very appropriate in these
situations.

Data-Driven DSS seem most appropriate where
managers need frequent access and ad hoc analysis of large
data sets. Model-Driven DSS are appropriate in recurring
decision situation that are semi-structured where a quantitative
model or models can inform or support analyses and choices.
Knowledge-Driven DSS are appropriate where a narrow domain
of expertise can be defined and where one or more experts
can be identified or where knowledge can be codified to help
a less expert decision maker. A Document-Driven DSS should
be built when a very large set of documents has been, is or will be
created that needs to be filtered, sorted, searched and analyzed.
A Communications-Driven DSS is most appropriate where two or
more people need to be involved in an ad hoc or on-going decision
process and either can't meet or find it costly to meet and
want to use technology tools to communicate, collaborate, evaluate
and support decision analysis or evaluation.