The sales force is usually the single largest line in a pharma commercial budget, and its size is set once and lived with for years. Get the method wrong and you either under-cover a reachable market or pay for reach you can never convert. Yet field-force sizing is too often done by analogy — "the last launch ran 80 reps, so we will too." There are three defensible sizing methods, and the right one is not a matter of taste: it is dictated by the commercial structure of the market.
The three methods
1. Workload (bottom-up)
Build the target list, decide the call frequency each tier deserves, divide by what one rep can physically do. Target accounts × frequency ÷ capacity = FTEs. Workload is the most rigorous method and the right default when the target universe is known and finite — a concentrated specialist market where you can name the accounts. Its weakness is that it assumes you should cover the whole list; in a market of hundreds of thousands of prescribers, workload sizing returns a number no one will fund.
2. Share-of-voice (competitive)
Size the force to match or beat the reach and frequency competitors put against the same prescribers. Share-of-voice is the right lens in a contested, promotionally-sensitive launch — typically primary care — where response tracks relative presence and the risk is being out-shouted. Its weakness is that it can start an arms race and says nothing about whether the spend pays back.
3. Affordable / ROI (top-down)
Start from the revenue the brand can realistically earn and work back to the force that revenue can profitably support. Affordable sizing is the right discipline under budget constraint or when promotional response is uncertain — it stops you buying reach you cannot convert. Its weakness is circularity: the forecast that sets the budget was itself built on an assumed level of promotion.
How the archetype picks the method
The decision collapses to one question: is the prescriber universe concentrated or diffuse?
- Concentrated / specialist (oncology, rare disease, most buy-and-bill markets) — the target list is small and nameable, so workload is the primary method. You are optimising account depth, not reach. Share-of-voice is secondary; affordability rarely binds because the force is small.
- Diffuse / primary care (cardiometabolic, respiratory, most pharmacy-benefit launches) — you cannot cover everyone, so affordable coverage sets the ceiling and share-of-voice sets the shape. Workload is used only to allocate the affordable force across deciles.
The same three methods, applied to two archetypes, produce completely different field models — which is why sizing cannot be copied between markets.
Worked example: NSCLC
Non-small cell lung cancer is a textbook concentrated market — a specialist base of roughly 13,000 US oncologists, buy-and-bill reimbursement, and biomarker-gated eligibility. That structure makes workload the governing method and an account-based key-account-manager plus MSL model the answer, not a mass-reach force. We work through the account tiering, channel coverage, and territory design in the NSCLC Field-Force Strategy analysis. A primary-care market of the same brand value would be sized the opposite way — affordable coverage first, workload only to allocate.
Common failure modes
- Workload in a diffuse market — returns an unfundable number; the team quietly overrides it with a budget cap and loses the rigour anyway.
- Share-of-voice in a specialist market — chases competitor rep counts when the real lever is account depth and testing rates.
- Sizing before segmentation — any method is only as good as the target list beneath it; decile the universe first.
AXLRx builds field-force sizing as a workload-based model with the archetype-appropriate method layered on top, scoped to your asset and accounts — delivered in 72 hours. See the NSCLC worked example or commission your own.