The Principle of Functional Cohesion shall be adhered to in all design, coding, inspection, and testing exercises. That principle is as follows:
A subroutine should perform one and only purpose and contain only entities necessary for that purpose.
This principle should be applied at all levels of abstraction. That is, you can substitute the word "module" or "form" for "subroutine", and the advice will be just as practical.
Adherence to this principle has been shown to result in systems that are modular, modifiable, understandable, testable, flexible, and delightfully maintainable. The occasional performance costs of this approach should be viewed as a reasonable price to pay for the increased maintainability unless evidence overwhelming proves that only an in-line approach meets customer needs.