Lessons · Project management · the 100% rule
The 100% rule: the children add up to the parent, exactly
Every level of the WBS contains all of the work of the level above and nothing more; the children of any box sum to exactly that box.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by an experienced project manager. Practice material, not professional advice. What that means.
What it is for
The budget rolls up from the work packages, so a package left out of the WBS is money left out of the budget, and it surfaces as an invoice nobody planned for. The 100% rule is the check that catches the missing box before it becomes the missing money.
How to think about it
At every box, ask two things: is there work in the parent that no child holds, and is there a child that is not really part of the parent. Then add the children's costs and compare them with the parent's.
Worked example
1.2 New building ready: $30,000The parent's figure.
1.2.1 $6,400 + 1.2.2 $9,600 + 1.2.3 $12,000 = $28,000The children add to $28,000.
$30,000 − $28,000 = $2,000 missingEither the parent is over-stated, or a child is missing. Look: nobody wrote signage installed. Add 1.2.4.
Your turn
Write the check the 100% rule asks at each box.
The children add to of the parent, no more and no less.
Solve one, graded on the server
The trap
Adding a miscellaneous child to make the numbers sum. It hides the missing package instead of finding it, and the invoice still arrives.