---
id: 2026-04-02T20:41:59-0400
title: 2026-04-02 20:41:59
tags:
  - status/draft
date-created: 2026-04-02T20:41:59-04:00
daily: "[[2026-04-02]]"
---
# 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.
