Furthermore, models are not always useful in analyzing non-cost factors in decision making. The manager, therefore, must recognize the capabilities and limitations of models and use them intelligently. Some of these limitations are now discussed. The problem of the instability of parametric models, as described by Boehm, is exacerbated by the sensitivity of effort and schedule to changes in input parameters. For example, in most cost models, changes in program size will result in at least an equivalent percentage change in cost or effort.
Some inputs have even more dramatic effects.
In SEER-SEM, changing security requirements from the lowest to the highest rating will have a similar effect. All models have one or more inputs for which small changes will result in large changes in effort and, perhaps, schedule. The input data problem is further compounded in that some inputs are difficult to obtain, especially early in a program. As discussed in section 19.4, size must be estimated early in a program using one or more sizing models. These models usually have not been validated for a wide range of projects.