Thursday, March 21, 2013

Estimates & Buffers

“A clever person turns great troubles into little ones and little ones into none at all.”

If you’ve been following the series on the well-formed schedule and the subsequent series of posts, you are aware that from the WBS you create estimates and iteratively refine those estimates by, for example, improving the probability of success and compensating for risk.  One of the PM challenges is how to balance motivating task owners, setting appropriate stakeholder expectations, reporting honestly and doing all of this in a forthright and ethical manner.

To demonstrate, let’s use our example task from Risk Buffers – An Example.  You might estimate that your Most Likely time for the commute is 30 minutes and the Optimistic time is 20 minutes.  However, the Expected (Mean) time might be calculated as 35 minutes (this is the 50% probability of success, where you’re as likely be early as late).  If you toss in one standard deviation to get to a 65% probability of success or two standard deviations to get to a 95% probability of success, the estimate could balloon to 45 or 55 minutes.  On top of this, then, you add those risk buffers, which might add 10-15 more minutes to the estimate.

Let’s take a look at this from your perspective with several of the stakeholders to examine the conflicts.  First, with the task owner:  Physics and Theory X management say that if you put that high estimate on the task, the task owner will take all of it.  In order to motivate the task owner, it is necessary to use, for example, either Most Likely or Expected.  In fact, I’ve managed projects in organizations that insisted that I use the Optimistic estimate and “stay on those team members” or they would just slack off.

Now looking at this from the perspective of your relationship with the project owner or sponsor, if you fill the schedule with fully bloated tasks (“What do you mean it’ll take 65 minutes to drive to the office!  I know full well it only takes 30 minutes.), you’ll lose all credibility.  However, if you use only Optimistic (!) or even Most Likely or Expected estimates, the end (date or cost) is not realistic and you’ve set yourself up for failure (as documented in The Schedule – Probability of Success). 

Some stakeholders want to know when it will really be done and how much it will really cost.  And they want to have realistic progress updates on these realistic completion values.  If you are not using fully weighted estimates, your reporting to these stakeholders will not be accurate.

Finally, another stakeholder that you have to true to is yourself (and the PM community).  How you address these conflicts must be ethical.

The conflict then is how to have individual task estimates that are under-weighted, but have the overall project estimate include the probability of success and risk adjustments.  The solution, of course, is to have “tasks” in the schedule that represent these estimating and risk buffers.  So let’s say you create these buffers and put them all at the end of the schedule.  This also creates reporting problems because there will be the appearance, at the start of the project, of a large gap between the end of the last deliverable and the end of the project.  It will appear as you progress and report completion of deliverables that you are (most likely) late on all of the deliverables.

At a minimum then, you need to distribute these buffers into each deliverable, spreading them over the life of the project.  You, as project manager, own these buffers and you have to deplete them as you go, continually reassessing much has been consumed by actual task overage and underage.
Since these buffers are included in the project (cost and time) budget, they cannot be arbitrarily reduced, eliminated, increased or created except through formal project change control.  They can (depending on the rigor of project change control) be moved between deliverables, but this should be documented.

Before closing I want to mention that the book Critical Chain by Elihayu M. Goldratt describes a formulaic approach to determine the appropriate amount and placement of the buffers.  This approach has been successfully used in many organizations and should be considered as an alternative to the custom and complex approach to buffers I’ve presented.  Further, he goes in to much more detail on the operational practices.
In my next post I’ll conclude this series on buffers and then we can move on to some new topics.

What challenges have you had convincing stakeholders to use realistic schedules?  Do you get pushback that you are just padding your estimates?  How do you handle it?
© 2013 Chuck Morton.  All Rights Reserved.

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