Configuring STREAMS

Maximum benefit of STREAMS is gained when two or more SEM programs are available. Regrettably, the cost involved in acquiring the programs is an obstacle even to the most avid modeller.

However, the good news is that the cost need not be at all this steep, and STREAMS may, indeed, be a cost-saver. This is particularly so if there is a network environment with multiple users of SEM programs. This is explained in greater detail below.

STREAMS has been designed with a client/server model in mind. The STREAMS program is the client, which implies that a copy of STREAMS executes on each active user's computer. The SEM programs are servers which are transparantly called by STREAMS when a model needs to be estimated. These programs may reside on any disk in a network, or on the local computer.

In a network environment there is typically a limit on the number of concurrent users of a program, but because the time spent estimating models typically is much less than the time spent editing models and interpreting modeling results the number of SEM programs licences can be considerably much lower than the number of STREAMS licenses. Thus, an optimally useful, and also inexpensive, configuration is one where, say, a dozen clients share a couple of network licences of each of the SEM programs.

In reality things are a little bit more complicated, however, because there are some further technical and commercial issues to be considered. These issues differ by program:

	EQS does not present any particular problems, and network versions for a varying number of simultaneous users are available. For example, a two-user version costs 347 US$ per year (see www.mvsoft.com).
	LISREL also is straightforward in this context. A network version costs 575 US$ + 85 US$ for each additional user. Thus, a two-user version costs 660 US$. The extra user licences are valid until there is a major upgrade of the program (see www.ssicentral.com)
	Mplus only is available as a single-user program according to the published price list, so it is left out of the present discussion. Observe, however, that it is possible to purchase multi-user site licences for this program too by contacting the office of Muthén & Muthén (see www.statmodel.com).