Your Estimate Isn't Off Because of Optimism or Inexperience — Julien Reszka
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The real reason your estimate is too low is not wishful thinking or a lack of experience. It is what you never named out loud.
45% Average estimation error when a professional team sized a real 1,340 hour software project as one whole, versus 23 percent after breaking it into parts first Jorgensen, M., Top-Down and Bottom-Up Expert Estimation of Software Development Effort, Information and Software Technology, 2004
Researchers asked Stanford students to estimate what share of all deaths came from different causes. When the causes were bundled into one category called unnatural causes, students estimated 32 percent. When the identical category was spelled out as accidents, homicides, or other unnatural causes, the estimate jumped to 53 percent. The real share of deaths from those causes did not change between the two questions. Only how the category was described did.
Amos Tversky and Derek Koehler built an entire theory out of that gap, now called the unpacking effect. A judgment attaches to how a category gets described, not to the event itself. Leave a category packed into one phrase and people quietly ignore the pieces they were never shown. Name the pieces out loud and the same people give it more weight, because now they are counting something they were previously skipping past.
The same pattern shows up somewhere with real deadlines attached.
Seven professional teams estimated a 1,340 hour software project as one whole and were off by 45 percent on average
Breaking the same project into its component parts first cut the error to 23 percent, roughly half
A second project showed the same pattern: 55 percent error estimated whole, 31 percent estimated in parts
Mapping the pieces of a plan before committing to a number works for the same reason naming the causes of death worked on those Stanford students. A single global estimate lets you skip past everything you did not name. Naming the parts, even the unglamorous ones, is what forces you to actually count them.
Myth: A number that comes in too low means the estimator was too optimistic, or not experienced enough.Jorgensen, M., Top-Down and Bottom-Up Expert Estimation, Information and Software Technology, 2004
“They indicate that probability judgments are attached not to events but to descriptions of events.”Amos Tversky and Derek Koehler, psychologists, in the paper introducing support theory
Before committing to an estimate, list the individual parts of the work by name, especially the ones you would normally lump into the rest. A number attached to a vague whole is quietly ignoring everything you did not name.
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Discussion
When you estimate a project, do you size it as one whole, or do you name every part first, including the ones you would rather skip past?
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See also
An Airport Sat Empty for $1.1 Million a Day
Bend Under Pressure, Not Under Nonsense
Your Exit Route Determines Your Strategy
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