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.
You can always use cost models
The young Asian woman is being drawn by an artist.
Gore tried to make Bush look better after the world affairs quiz.
I don't know how to read.
The workers are building cars.
A man is playing the saxaphone
She claimed to not have had sex in a couple of years but I wasn't convinced.