Guidance on how much can be withdrawn annually from a retirement portfolio is usually expressed as a single percentage. The disagreement about that figure concerns the assumptions underneath it.
The question being answered is narrow
The rules attempt to find the largest initial withdrawal, increased each year with inflation, that would have survived every historical sequence of returns over a fixed retirement length.
That framing fixes several things at once: a constant real spending pattern, a specific portfolio mix and a defined horizon, typically around three decades.
Change any of those inputs and the answer moves, which is why figures quoted with apparent precision differ between analyses that all appear reasonable.
The historical record used determines the answer
Studies drawing on one country's long-run market history produce higher sustainable rates than those using a broader international sample.
The difference is not a methodological error. Some markets experienced interruptions and long real declines that a single favourable history does not contain.
Whether the favourable record represents the future or an unusually good sample is unresolvable, and it is the core of the dispute.
Starting valuations shift the odds
Retirements beginning when assets are expensively valued have historically supported lower withdrawals than those beginning after a decline.
The reason is that valuation influences subsequent returns, and the early years of a drawdown matter disproportionately to whether the portfolio survives.
A fixed rule ignores this by construction, applying the same rate regardless of the conditions at the moment retirement starts.
Real spending is not flat
The rules assume constant inflation-adjusted spending for life. Actual retirement spending tends to fall through the middle years and rise again with care needs.
A retiree who adjusts spending downward in poor market years can sustain a higher starting rate than one committed to a fixed real amount.
Flexible approaches accept a variable income in exchange for a lower chance of depletion, which is a different trade rather than a better answer.
Longevity is the input least often examined
A rate calibrated to a thirty-year horizon says nothing useful to someone retiring early or to a couple where one partner may live considerably longer.
Planning to an average lifespan means roughly half of retirees outlive the assumption, which is why guaranteed income sources are treated separately from portfolio withdrawals.
Because these variables interact and personal circumstances dominate, drawdown planning is an area where individual professional advice does substantive work.