Which assumptions are actually carrying the investment case?

Which Assumptions Are Actually Carrying the Investment Case?

A headline return can be supported by robust operating improvements or by a narrow combination of growth, price, timing and terminal value. The number alone does not reveal fragility. An investment case is strong when no unsupported assumption has enough leverage to reverse the decision.

Decompose value into operational drivers: volume, ramp, price, yield, uptime, variable cost, capital, working capital, life and residual value. Trace each to a source and owner. Then calculate elasticities and switching values�the change required for the preferred option to become inferior or destroy value. Rank assumptions by decision impact and evidential weakness.

One-at-a-time sensitivity is only the start. Build coherent scenarios in which related variables move together: weak demand can reduce price and delay ramp while leaving fixed cost unchanged; inflation can raise construction, wages and financing simultaneously. Model schedule as an economic variable because delay often shifts benefits later while cost and committed capital continue.

The 2026 Green Book requires sensitivity analysis and switching values for preferred options and recommends explicit adjustments for historical optimism in cost, benefit and duration. This discipline turns debate from whether a forecast feels conservative to whether the evidence can withstand the values at which the decision changes.

Maintain an assumption ledger with baseline, range, evidence age, correlation, owner and trigger. Spend diligence on the few assumptions carrying most value and design stages around their resolution. Update the case when evidence moves, without hiding the old view. A resilient investment earns its return across a credible range rather than at one carefully balanced point.