Scenario modelling

Scenario modelling is the practice of writing down multiple possible futures as explicit numerical projections, then comparing them side by side to inform a decision. Rather than choosing the “best” outcome, scenario modelling makes the trade-offs between options visible — which is usually more useful than a single point forecast.

How it works

Each scenario is defined by a small number of inputs: a starting position, a recurring monthly change (contribution, payment, or withdrawal), a growth or interest rate, and any one-time costs. Each scenario is projected forward year-by-year using the same horizon, and the resulting trajectories are compared.

Three kinds of comparison emerge:

• Final-value gap. Which scenario produces the highest end-of-horizon balance, and by how much.
• Crossover year. When a scenario that started behind overtakes a scenario that started ahead. This is often the most decision-relevant insight, especially for buy-vs-rent or save-vs-invest questions.
• Path shape. Whether one scenario is steady while another is back-loaded — material for liquidity planning even when final values are similar.

When scenario modelling helps

It’s most useful for decisions where:

• The math involves long horizons (10+ years), where compounding makes intuition unreliable.
• Multiple variables interact — return, contribution, fees, time — so back-of-envelope comparison breaks down.
• You’re comparing structurally different options (rent vs buy, debt vs invest), where the “winner” depends on which assumptions you trust.

What it doesn’t do

Scenario modelling is not a forecast. It does not predict the future, model uncertainty, or account for behavioural factors. The output is only as good as the inputs — which is precisely the point: it forces the assumptions out into the open where they can be examined and adjusted.

A common discipline is to run each scenario twice: once with realistic assumptions, once with assumptions 1–2 percentage points worse. If the same scenario wins under both, the decision is robust. If the winner flips, the decision is closer than your gut suggests.