9.3 Modelling software and simulations
The software package we used is called Ithink Analyst.  Four key element types are used to build Ithink models.
A stock represents an accumulation. The items accumulate by flowing into and/or out of the stock (see description of the `flow' below). The total content amounts to the inflow minus the outflow at each time period in a model simulation. In many of the stocks represented in our models the inflow and outflow are equal. For example, a journal editor receives a number of manuscripts every year. Of those, he or she rejects a very small percentage and the remainder are sent for peer review. The same number of manuscripts enter and leave the editorial office.
A flow either fills or drains a stock in the direction of the flow arrow. A cloud at either end of a flow indicates an infinite source of or destination of the material flowing to or from a stock. Basically this indicates that the source of material passing through the flow is beyond the scope of the model.
A converter informs other elements in the model. It may contain a constant value, e.g. tax at 17.5%; an incremental value, e.g. 1 in year 1 and rises by 1 in each subsequent year; a variable which can be manipulated by a model user; or an algebraic relationship between different elements in the model.
A connector is like a wire which transmits information between elements in a model, e.g. in Figure 9.1 the flow labelled `xfer to ref' represents the number of manuscripts that are sent to referees to be reviewed. The value of this flow is determined by the number of manuscripts received by the editor (MS received), and the number rejected immediately, e.g. because the subject is unsuitable. The value of the converters is conveyed to the flow by connectors.
Figure 9.1: A chunk of model consisting of one stock, three flows and two connectors.
Each of our models consists of four interconnected sectors: content origination, publication, information brokerage, and the library function. The models all simulate production of a small journal which publishes 120 10-page papers per annum. We used Ithink to represent graphically the interrelationships that characterise each system. We then defined numerically each element in the model. Some of these definitions are equations which describe the relationship between two or more elements in the model. The bases of the equations and the assumptions in each model element are described within the model in element `documents.'  These can be viewed by a model user. The models are designed to be used rather than viewed. Although we deliberately kept them as simple as possible, the systems modelled are fairly complex. Pictures of whole models cannot be captured in a page.
Figure 9.2 is an example of the publisher section from the market model. It includes two stocks: "publication" and "publ budget". "Publication" is the accumulation of articles published in the journal. The flow from the "origination" sector into this stock is not shown. "Publ budget" is the publisher's budget. Costs ("publ spend") and profit ("publ profit") flow out of it and revenues flow in. The flows representing revenues from other sectors are not shown in Figure 9.2. Converters are used to calculate various values in the model. For example, total publishing costs are calculated with reference to publication costs, editorial costs and the overhead applied to those. The total publication costs informs the authors' contribution, i.e., the value of the author fee. Converters with rectangular buttons in them (e.g. "overhead" and "profit margin") represent those whose values are determined by the model user.
Figure 9.2: The publisher sector of the `Market model'