If S1 is the total number of modules defined in the program architecture, and S3 is the number of modules whose correct function depends on prior processing, then the number of modules not dependent on prior processing is:

Explanation

This is the D3 term of Pressman's Design Structure Quality Index (DSQI), which is computed from the architectural counts S1 to S7. S3/S1 is the fraction of modules that do depend on prior processing, so the complementary fraction — modules independent of prior processing — is 1 – (S3/S1). Adding the ratio (options 1 and 3) would give a value greater than 1, which cannot be a fraction of modules, and inverting the ratio (options 3 and 4) divides by the wrong total. A high D3 is desirable because modules that do not depend on prior processing are easier to reuse and test in isolation.