4pm London fix: the most overrated and underrated thing at once

[Shadow Trader]

The WM/Reuters 4pm London fix is the benchmark many funds use to value their portfolios. Month-end, that creates a lot of rebalancing flow, and traders love to talk about the volatility around it.

Overrated: the idea that every day has a tradable pattern into the fix. Most days, nothing special happens. Price wobbles a few pips and moves on.

Underrated: month-end, quarter-end, and big rebalancing days. When equities have moved sharply during the month, funds with foreign holdings have to hedge or rebalance their currency exposure. That flow can arrive in a concentrated burst in the final half hour before 4pm.

On those days, the move can be one directional, sizable, and quick, followed by a partial reversal once the window closes. Scalpers who ignore the calendar get surprised. Scalpers who look at it can at least avoid fighting the flow.

The tricky part is knowing which month-ends matter. A sleepy month with flat equities produces little. A month with a large equity swing can produce a violent fix.

A cleaner way to study it is to pick the last three business days of each month and compare the 15:30 to 16:05 London window with the same window mid-month, across a couple of years of tick data. Measure the average absolute move, the spread, and whether the direction in the final fifteen minutes before the fix tends to reverse afterward.

The results usually show a visible difference only on a minority of days, which is the point. The fix matters on the days when it matters, and a calendar filter tells which those are. Knowing that, a scalper can step aside on the heavy days and trade normally on the rest, instead of treating the fix as a permanent hazard.

How much attention does the fix deserve outside of month-end?