Integration tends to take more time than expected, so ensuring the system works end-to-end in the beginning of the project avoids surprises and schedule slips late in the project. In other words, it's one of the Strategies for attacking project risk.

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References

James Hamilton Get the skeleton version of a full system up early in the development cycle. It gets developers thinking at the user level from the very beginning.

‣ Tracer Bullets

Build end-to-end, not top-down or bottom-up.