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+---
+id: 2026-04-02T20:41:59-04:00
+aliases: []
+title: 2026-04-02 20:41:59
+tags:
+  - authorship/original
+  - destiny/permanent
+  - status/draft
+  - type/periodic/timestamped
+dg-publish: true
+date-created: 2026-04-02T20:41:59-04:00
+daily: "[[2026-04-02]]"
+weekly: "[[2026-W14]]"
+monthly: "[[2026-04]]"
+quarterly: "[[2026-Q2]]"
+yearly: "[[2026]]"
+---
+# 2026-04-02 20:41:59
+
+## Decomposition Fallacy
+
+I have expressed in other notes[^1] my frustration
+with how estimate decomposition is most frequently explained:
+as an intuitive process which can not be performed incorrectly
+and by definition creates a more accurate estimate.
+
+[^1]: * [[2026-01-25_21-02-00]]
+    * [[2026-01-25_22-59-00]]
+
+The argument that an estimate based on the aggregation of intermediate estimates
+is necessarily more reliable than one made in one go
+is intuitive, but a [non sequitur](https://en.wikipedia.org/wiki/Formal_fallacy).
+
+Frequently, decomposition is explicitly _unhelpful_
+due to its neglect of the [portfolio effect](https://financial-dictionary.thefreedictionary.com/portfolio+effect).
+If an estimator can give a high confidence estimate
+of total dollars spent on rental equipment over the life of a project,
+it does not follow that they could give one for
+dollars spent on scissor lifts alone with greater or even equal confidence.[^2]
+
+[^2]: I chose this example specifically
+    because it has been an issue at both EC's I've estimated for.
+    We set a target based on some well founded model
+    (\$0.25 per direct labor hour or 1% of total sell price),
+    then 3--4 decently paid estimators spend up to an hour
+    discussing how to make that target
+    from quantities and durations of individual pieces of equipment.
+    Ultimately none are totally satisfied with it, nor should they be,
+    since the actual quantities, durations, and equipment types will be quite different,
+    and the entire exercise was pointless.
+
+My criticism is not entirely fair, however.
+Fermi and MacGregor's estimation problems have correct answers
+and are thus a different class entirely
+from those I'm most familiar with.
+I choose to criticize decomposition generally despite this
+because the distinction is rarely acknowledged by its advocates
+so "decomposition = good" pervades the estimating zeitgeist completely
+and I get baffled, horrified, or patronizing looks when I suggest
+that complete quantity takeoff may not be more accurate
+than a parametric budget that could be completed in a day.