Table 12 Sept 2026From “What are TAM, SAM, and SOM?”

Why do top-down market sizing models fail in commercial execution?

Table 1Why do top-down market sizing models fail in commercial execution?

Dimension	Top-Down Sizing (Analyst Research)	Bottom-Up Sizing (Unit Operations)
Primary data source	Third-party industry analyst reports (Gartner, IDC)	Company CRM data, firmographic registries, lead lists
Core calculation logic	Applies speculative percentage haircut to macro industry	Multiplies verified account counts by realized deal size
Addressable boundaries	Assumes universal product compatibility	Excludes accounts lacking required integrations or scale
Sales capacity constraint	Ignored; assumes infinite commercial execution	Directly bounded by sales rep ramp, quota, and deal velocity
Competitive displacement	Assumes greenfield capture	Models incumbent switching costs and contract lock-ins
Decision relevance	Useful for venture capital storytelling	Mandatory for territory design, hiring, and revenue quotas
Typical failure mode	Massive overestimation of reachable revenue	Can be overly conservative if new channels are omitted

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Source: Table from this essay. Sources and interpretation are given in the article.

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From the essay

Go-to-market strategy

What are TAM, SAM, and SOM?

TAM, SAM, and SOM establish a nested market sizing architecture that separates aggregate theoretical demand from operational go-to-market capacity.

2 Sept 2026

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Isoglu, S. (2026). Why do top-down market sizing models fail in commercial execution? [Table]. isoglu.com. https://isoglu.com/insights/exhibits/isoglu-2026-what-are-tam-sam-som-fig02/

BibTeX

@misc{isoglu-2026-what-are-tam-sam-som-fig02,
  author = {Isoglu, Sinan},
  title = {{Why do top-down market sizing models fail in commercial execution?}},
  year = {2026},
  howpublished = {Table 1, isoglu.com},
  url = {https://isoglu.com/insights/exhibits/isoglu-2026-what-are-tam-sam-som-fig02/},
  note = {From the essay {What are TAM, SAM, and SOM?}}
}

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  <figcaption>Table 1: Why do top-down market sizing models fail in commercial execution?. Source: Table from this essay. Sources and interpretation are given in the article. <a href="https://isoglu.com/insights/exhibits/isoglu-2026-what-are-tam-sam-som-fig02/">View exhibit</a>.</figcaption>
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