In Part One of this blog I made a strong case as to why using high-end task duration estimates is not the way to go when scheduling your project. High-end estimates extend project timelines in both planning and practice. To recap, implications of using high-end estimates are:
1) Project end dates are greatly extended from detailed planning, driving managers nuts. AND,
2). Safety (time) intended to protect the project from uncertainty is used up whether needed or not, pushing out project completion.
Therefore, we turn to the other two estimating alternatives — low-end and middle (or median). Remember the most sophisticated way to determine a task duration median is to use this calculation:
(high-end + 4 x mean + low-end) / 6
Conventional wisdom is summarized like this: “Using low-end estimates assume everything is going to go well and that’s just crazy sauce. Let’s use this calculated median method.”
The idea that a task duration estimate will fall within a skewed distribution is noble (and quite correct). If one were to repeat the same task 100 times (like commuting home from work), you’ll see results that show a distribution, skewed to the right.
Durations of an identical task, executed repeatedly.
But selecting a SINGLE NUMBER to represent this distribution again leads to unintended negative consequences.
The thinking behind using a median estimate is that half the time the task will complete early, and half the time late, therefore it’ll all balance out. The reason this assumption is often wrong is because work expands to fill the time allotted and people deliver their tasks ON their commitment dates, not early (the same concern over using high-side estimates). In practice, median-estimated tasks will complete ON TIME half the time and LATE half the time. This is a law of human nature. This also means projects will likely be delivered late when using median estimates.
Well where does that leave us? Low end estimates? There’s NO safety to deal with uncertainty. That can’t be right, can it?
This is where the BIG BREAKTHROUGH happens… check out Part Three.
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