The worst error in scheduling is to have a milestone depend upon two or more tasks scheduled (planned) to finish at the same time.Here's a footnote to that witticism: It's assumed the two tasks are independent, meaning:
- They don't share resources
- They don't have the same vulnerabilities to a common risk
- The progress, or not, in one does not affect progress in the other
So, what's the big error here?
- First, as regards milestone success , each of the tasks leading into the milestone is a risk to success (success means: it is achieved on time)
- Second, total risk is the product of all the input risks (as seen by the milestone) .
- So, whereas each task coming into the milestone may not be too risky, say by example 90/10 (*), three tasks of 90/10 each would present a risk to the milestone such that success is reduced to about 73/27
(*) 90 successes out of 100, or 90% chance the task will finish on time, or early.
What are you going to do about this?
Bring on the time buffers! (**)
- You might be able to add a buffer on one or more of the input tasks to raise the success of that task to 99/01, or so
- You might be able to add a buffer following the milestone, such that any late success is absorbed by the buffer (This tactic is called "shift right" by schedule architects)
- You might be able to reorganize the schedule to eliminate this milestone, or one or more of its input tasks.
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