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A single 5-year number tells you what one lucky start date produced. Rolling returns test every start date — so you see the average, the best window, the worst window, and how often the fund actually cleared your target.
A rolling return is an annualised return measured over a fixed-length window that slides through history. Instead of asking "what did this fund return over the last five years?", it asks the same question of every month in the period — producing a distribution rather than a single figure.
That distribution is where the useful information lives. Two funds can show the same 5-year trailing return while one delivered it steadily and the other swung between −4% and +34% depending on when you entered. Rolling returns separate consistency from start-date luck.
It is the return of the unluckiest investor. If you cannot live with that number, the fund is not right for the goal.
This is the fair planning assumption — closer to reality than a trailing number picked from one date.
Best minus worst shows how much entry timing mattered. A tight spread means a dependable fund.
Use the window that equals your intended holding period, and prefer funds that clear your target in most windows.
Rolling returns are annualised returns measured over every possible start date in a period, rather than from one fixed date. A 5-year rolling return computed over 15 years produces roughly 120 overlapping 5-year windows, each showing what an investor who started in that month would have earned per year.