53 lines
2.2 KiB
Markdown
53 lines
2.2 KiB
Markdown
---
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id: 2026-04-02T20:41:59-0400
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title: 2026-04-02 20:41:59
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tags:
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- status/draft
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date-created: 2026-04-02T20:41:59-04:00
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daily: "[[2026-04-02]]"
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---
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# 2026-04-02 20:41:59
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## Decomposition Fallacy
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I have expressed in other notes[^1] my frustration
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with how estimate decomposition is most frequently explained:
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as an intuitive process which can not be performed incorrectly
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and by definition creates a more accurate estimate.
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[^1]: * [[2026-01-25_21-02-00]]
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* [[2026-01-25_22-59-00]]
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The argument that an estimate based on the aggregation of intermediate estimates
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is necessarily more reliable than one made in one go
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is intuitive, but a [non sequitur](https://en.wikipedia.org/wiki/Formal_fallacy).
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Frequently, decomposition is explicitly _unhelpful_
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due to its neglect of the [portfolio effect](https://financial-dictionary.thefreedictionary.com/portfolio+effect).
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If an estimator can give a high confidence estimate
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of total dollars spent on rental equipment over the life of a project,
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it does not follow that they could give one for
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dollars spent on scissor lifts alone with greater or even equal confidence.[^2]
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[^2]: I chose this example specifically
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because it has been an issue at both EC's I've estimated for.
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We set a target based on some well founded model
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(\$0.25 per direct labor hour or 1% of total sell price),
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then 3--4 decently paid estimators spend up to an hour
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discussing how to make that target
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from quantities and durations of individual pieces of equipment.
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Ultimately none are totally satisfied with it, nor should they be,
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since the actual quantities, durations, and equipment types will be quite different,
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and the entire exercise was pointless.
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My criticism is not entirely fair, however.
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Fermi and MacGregor's estimation problems have correct answers
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and are thus a different class entirely
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from those I'm most familiar with.
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I choose to criticize decomposition generally despite this
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because the distinction is rarely acknowledged by its advocates
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so "decomposition = good" pervades the estimating zeitgeist completely,
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and I get baffled, horrified, or patronizing looks when I suggest
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that complete quantity takeoff may not be more accurate
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than a parametric budget that could be completed in a day.
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