Emphasis on "Being Done" in Agile - 90% Syndrome (Mike Cohn)

Courtesy - Mike Cohn

• We often fail to gauge the magnitude of an effort until we are well into that effort. 
• For this reason, conventional estimations are not quite accurate.
• Ask a developer how "Done" something is and you get to hear "90% complete". A week later ask him again, and you get the same reply "almost 90% complete". This happens because the developer has gauged the scope of work incorrectly. He fails to anticipate all that is needed to complete the work.
• The 90% syndrome means the developer is certainly making progress, however is progressing at exactly the same rate as his understanding of the problem's scope.

Example

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Microsoft's development of MS Word began in September 1984 and was estimated to take 12 months. Nine months later, the team realized it will take another 13 months to complete, and an year later the team estimated 11 months. 

For three years, MS Word was estimated to be an year away. The product was ultimately shipped 5 years and 3 months later.