Why the Measurement Window Changes a Fund's Rank

One fund, five windows, five different stories

Every return and risk metric is computed over some window - 1 year, 3 years, 5 years, and so on - and the window you choose changes the answer, sometimes drastically. This is not a flaw in any single calculation; each window is correctly computed. The issue is that a fund’s category percentile on a 1-year window can look nothing like its percentile on a 5-year window, because a different set of months dominates each calculation.

SBI Focused’s rolling CAGR percentile, by window

On the 1-year window, SBI Focused ranks 2 of 28 in its category, the 96.43th percentile - near the top. On the 3-year window, the same fund ranks 14 of 27, the 51.85th percentile - right around the middle. The category rank swings by 44.6 percentile points depending purely on which window you look at. Nothing about the fund’s strategy, manager or holdings changed between those two numbers; only the measurement period did.

Risk-adjusted return swings even more over short windows

The Sharpe Ratio here ranges from -0.31 on the 9M window to 1.59 on the 6M window. Short windows (3, 6 and 9 months) are the most volatile, because a single good or bad quarter can dominate a short calculation. The 7-year and 10-year figures, by contrast, sit closer together (0.66 and 0.56), because a longer window averages over more market cycles and is less sensitive to any single stretch.

The general pattern

Short windows are noisy and can be dominated by a single sharp move. Long windows are smoother, but can hide a recent change in strategy or manager, since years of old performance still weigh on the average. There is no single “correct” window; the right one depends on your own holding period. If you plan to hold a fund for three years, its 3-year numbers are more relevant to you than its 1-year or 10-year numbers, even if those look more flattering.

How to use this when comparing funds

Always state the window when you quote a rank, a percentile, or a risk-adjusted ratio, and compare two funds on the same window. A fund that looks excellent on a 1-year window and mediocre on a 3-year window is not necessarily getting better or worse; it may simply have had one strong recent stretch. Check more than one window before drawing a conclusion, and weight the window closest to how long you intend to actually hold the fund.

Frequently asked questions

Which window should I trust most?

None by itself. Look at several windows together, and give the most weight to the one closest to your own intended holding period. A fund that ranks well across multiple windows is more convincing than one that ranks well on only one.

Why do short windows like 3-month Sharpe swing so much?

Because they are computed from very few data points, so a single unusually good or bad stretch of weeks can dominate the whole calculation. Longer windows average over far more periods and are correspondingly steadier.

Does a fund’s window-to-window swing mean something is wrong with it?

Not necessarily. Some swing across windows is normal for every fund, since markets move in cycles that do not line up neatly with any fixed window length. A very large, erratic swing across every window pair is worth investigating further, but a moderate difference between, say, 1-year and 3-year figures is ordinary.