Also, they are usually fast and easy to use, require little detailed information, and capture system or CSCI-level costs. The STSC Report adds that the results can be questionable when estimating future projects that use new technologies. Also, the formulas generally are unable to deal with conditions such as exceptional personnel, exceptional teamwork and exceptional matches between skill levels and tasks. Parametric modeling is usually the methodology of choice for Government agencies because of the minimal time and effort required and because it is useful in early phases of a software development program when little is known about the program. Also, attempts are continually being made to calibrate available models to insure greater accuracy.
All parametric models use one or more design parameters to estimate cost or level of effort.
The earliest parametric models, some of which are still in use, were simple models that estimated cost based only on the size of the software in SLOC, machine instructions, or other measures. The coefficient and exponent are usually computed using mathematical regression analysis on historical size and effort data. This simplistic model generally shows considerable variance, even within the database from which the model was formulated, which is one reason why simple single-parameter parametric models, such as those based solely on program size, are not generally considered to be accurate. Another limitation of these models is that the coefficients and exponents vary significantly with the database used, indicating that the models are highly database dependent. This suggests that other factors, besides the size of the software program, affect the effort or cost required.