Conscientious use of constrains

Generally, constraints affect the dates, violates the network logic and one of the reasons for negative floats or irrationality on the schedule. But again this is based on your project requirements and these constraints could help you to provide a complete control of a project however no more than 10% of a project’s activity should be constrained. 

Constraints other than contractual ones are generally not used since they impose specific start and or finish dates and in some instances completely OVERRIDE the logic that is contained in the schedule. The idea is to let the logic determine the dates as opposed to using constraints to do this.

Early constraints (e.g. Must Start/Finish On or After) are a typically a shortcut to represent the outcome of related work that the scheduler is excluding from the schedule logic. This is justified only for interfacing work that - in total, including its initiation - is clearly outside the scope of the project scheduled (e.g. contractual access restrictions, customer-furnished info/eqpt/permits.) Using early constraints for in-scope work (or for external work that depends on in-scope work) removes that work from logical schedule analysis, overrides the logically-derived scheduled (early) dates of the constrained activities and their successors, and jeopardizes the validity of the entire logical model of the project including float calculations.

 Late constraints (e.g. Must Start/Finish On or Before) are typically imposed to represent external obligations or commitments – aka “deadlines.” Such commitments can be imposed by contract (e.g. completion milestones) or by some other governing document (e.g. Project Charter, Board Instruction, Executive Tantrum, etc.) Late constraints can override the logically-derived (late) dates for the constrained activities and their predecessors, thereby complicating the interpretation of Total Float and identification of the “Critical Path.” Where multiple late constraints are applied in a network of related activities, Total Float becomes unreliable as an indicator of driving logic; then other methods of logical analysis must be used.

Other constraint types (Start/Finish On, or Mandatory Start/Finish) are even more restrictive with respect to driving logic flow – they are rarely if ever justified.

It is true that ALL constraints affect total float computations but so do the calendars being used, the remaining durations, the logic, the lags, the TIMES etc. So I would re-iterate that this clause was included in the scheduling specifications because constraints impose specific start and or finish dates and in some instances completely OVERRIDE the logic that is contained in the schedule.