Another new feature in the NETFLOW procedure is that it enables you to specify a network containing both nodes with missing S supply values and nodes with missing D demand values. This feature is a powerful modeling tool, and we show in the later sections how to use it to formulate and solve maximum flow problems and network models with range constraints on supply and demand.

Whenever a network is detected to have both nodes with missing S supply values and nodes with missing D demand values, a special value of the EXCESS= option is assigned internally by the procedure; any value you specify for the EXCESS= option is overridden. The procedure solves the problem in the following manner:

Figure 7.28 displays how the slack variables are set up by the procedure internally. These variables are neither a part of the input data set nor displayed in any output SAS data set or printed output.