While the layout of a WBS doesn’t need to be perfect, the content does. In other words, you absolutely have to account for 100% of the work scope of a project. You must strive to not leave anything out. If you do miss something that is discovered later, you’ll have to ask for additional money and/or time to perform that missing scope, you will have to draw upon contingency reserves to pay for it, and/or you’ll have to sacrifice other, lesser important work scope to pay for the newly discovered scope. None of these are good options.
One way to find the entirety of project scope is via the application of Progressive Elaboration. Hit the higher-level WBS elements first. Review to ensure you have all the big-picture scope accounted for, then drop down a level and subdivide and breakdown the scope. Rinse and repeat at each progressively lower level of the WBS until you get to the work package level. Visualize how a subsystem comes together; what are its constituent components, and what is and isn’t included. Start big and decompose.
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You can also email me directly at Mark@TheProjectManagementBlueprint.com