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Retirement Planner · Track 3
R3 of 3 — Result guides

The what-if comparison

The what-if comparison is where the planner becomes a decision tool. Save a baseline, change something, run again — and see the exact cost or benefit of that change in probability terms.

CHANGED PARAMETERS Retire age 55 58 ↑ Monthly contributions £5k £7.5k ↑ CPI rate 2.5% 2.5% Withdrawal strategy Cash buffer band Horizon 30 yrs OUTCOME METRICS Success probability 83.6% 91.2% +7.6pp P50 final balance £1.40m £1.87m +£470k P10 final balance £0 £124k +£124k Median shortfall months 3 mo 0 mo −3 mo Baseline Current Change
How to set up a fair comparison
1
Run your baseline simulation first

Configure your plan and run the simulation. This becomes Scenario A — the plan you are comparing against.

2
Click Save as baseline on the What-if tab

In the Results panel, click the What-if? tab and click Save as baseline. The simulation is locked and the random seed is recorded.

The locked seed is fundamental to fair comparison. Without it, any difference between runs could be random variation in the market paths rather than the effect of your input change. With it locked, every difference is purely from what you changed.
3
Change one thing at a time

Go back to the inputs and change one variable — retire age, contributions, withdrawal strategy, an expense, a crisis scenario. Change one thing so you can attribute the result clearly.

Changing multiple inputs at once makes it impossible to understand which change drove which outcome. Use one change per comparison, then save the improved version as the new baseline and make the next change.
4
Run again and read the comparison

The What-if tab shows the changed parameters table, four comparison charts and the metrics table. Changed parameters are highlighted in blue.

Reading the comparison output
5
Changed parameters table

Every input that differs between baseline and current run is highlighted. Unchanged inputs are shown in grey for context. This confirms you changed what you intended to change — and nothing else.

6
The four comparison charts

Portfolio fan shows both runs across the full horizon. Success probability shows the probability curve over time for both. Cash buffer P50 shows the median cash path. Final balance distribution shows P10–P90 end balances as grouped bars.

Read the charts as a narrative: where do the paths diverge? If retiring 3 years later, the divergence starts at the retirement line — the longer pre-retirement period builds a higher portfolio.
7
The metrics delta

Green delta = current run is better on that metric. Red = worse. The deltas are absolute — +7.6pp means 7.6 percentage points, not 7.6% of the baseline value.

Practical what-if questions to ask
8
What is the cost of retiring 2 years earlier?

Save your current plan as baseline. Change retire age from 60 to 58. Run. Read the success probability delta — this is the exact cost, in probability terms, of those two years.

9
What does an extra £500/mo in contributions buy me?

Save baseline. Increase monthly contributions by £500. Run. The P10 final balance delta shows how much the worst-case outcome improves — often more telling than the P50.

10
Can my plan survive a 2008 crash in year 3?

Save the no-crisis run as baseline. Add the 2008 crisis scenario at timing = year 3. Run. The success probability delta tells you exactly how much that sequence of returns costs you.

You have completed all retirement planner guides.

You now understand both the mechanics and the reasoning behind every input and output. Open the planner and put it to work.

Track 3 · R3 of 3