Using cost models to estimate the cost and length of time necessary to develop a new software system, for example, is appropriate only after requirements have been defined, a system design has been developed, and the size of the new system has been estimated.
You can only use cost models in certain circumstances.
mnli hypothesis: You can only use cost models in certain circumstances. premise: Using cost models to estimate the cost and length of time necessary to develop a new software system, for example, is appropriate only after requirements have been defined, a system design has been developed, and the size of the new system has been estimated.