Hard vs Soft Dependencies

Yet another way to look at dependencies is to rate their strength of connection. Some connections are physical, that is, it is physically impossible for activity B to start until activity A finishes. For example, it is impossible for the "allow paint to dry" activity to occur before the "apply paint" activity begins. And gravity prevents us from doing the "install roof activity until the "erect walls" activity finishes. Similarly, we cannot achieve the "code compiled" milestone until the "write code" activity finishes, and we cannot start the "review design specification" activity until the "draft design specification" at least begins. These connections are said to be "hard logic" or "mandatory dependencies." They cannot be broken, and so the activities involved must be sequential, lengthening the total project duration.

However, some dependencies are discretionary, and may allow activities to overlap. An example would be that "paint living room" and "paint bedroom" could occur simultaneously. Often, it is not obvious that the dependency is discretionary. For instance, to non-builders it first appears that "build slab foundation," "build walls," and "build chimney" are sequential hard-logic activities. However, most construction managers have learned that framing the walls can begin at the same time as foundation construction by building them laying down on the ground next to the slab. Then when the slab's concrete cures enough to walk on, they can erect the pre-built walls onto the slab and nail the corners together. Software projects are full of these discretionary dependencies. For example, "draft design specification" need not wait to start until after "complete requirements document" has finished. It may start at the same time, or lag slightly behind, thus allowing the schedule to be compressed a bit. These so-called "soft-logic" dependencies are the project manager's key to constructing an aggressive, but realistic, schedule.

Hard and soft dependencies are summarized in Box 14?1.

Box 14-1 Dependencies by Strength of Connection


Also Known As



® Mandatory dependencies

A strong connection where an activity cannot

start until a preceding one is completed

® Logical



® Discretionary dependencies

® Preferred logic

® Preferential logic

A weak connection between activities that is defined by the project team according to best practices or judgment

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