What is the expected move — and why is yours too narrow?
The expected move is the band an underlying is implied to stay inside over a given period, and it is the single most useful number an option seller can look at. It is also, in almost every tool that prints it, systematically wrong in the same direction.
How it is normally calculated
The standard method takes at-the-money implied volatility, scales it by the square root of time, and quotes one standard deviation. On a normal distribution that band covers roughly 68% of outcomes, so the number gets labelled a 68% range.
A common shortcut uses about 85% of the at-the-money straddle price instead. Both land in the same place, and both inherit the same assumption: that daily returns are normally distributed.
Real days do not behave like a bell curve
Markets have fat tails. Big days happen far more often than a normal distribution allows, and the effect is strongest in single names rather than broad indices.
The consequence is specific and costly: a band derived from the normal assumption is too narrow, so it breaks more often than the label on it claims. If you sell strikes against a 68% band that is really a 60% band, you are being paid for a risk you have mismeasured.
Measuring it instead of assuming it
Rather than assume a shape, you can measure the actual distribution of daily excursions and read the band straight off it. Do that and the difference shows up immediately on individual stocks — a measured band on a high-beta name can run materially wider than the textbook figure, while broad ETFs sit close to it.
FlowMonkey's containment band on Compass is built this way: inverted from a measured curve rather than a normal distribution, and re-priced through the session as time runs out. It is also deliberately asymmetric, because price reaches down slightly more readily than it reaches up at the same distance.
How to use it without fooling yourself
Treat the band as containment, not prediction. It answers where price probably will not go, which is a genuinely different and more answerable question than where it will.
And check the horizon. A band quoted for a full session means nothing at 3pm — most of the day's travel is already spent, and the honest band by then is a fraction of its opening width.
The expected move is worth watching and worth double-checking. If your tool derives it from a bell curve, assume it is optimistic on single names.
Keep reading
See it on a live board
FlowMonkey prices every level by whether your ticker can actually reach it today — measured on that symbol's own range, re-sorted as the session burns down.
See pricing