You have to include the previous close, or it isn't the so-called "true" range, it's just plain real range.Quote from JackR:
The thread is entitled " Average True Range" Thus, "ATR".
If you mean what is ATR, it is the average of the true range (TR) of the maximum difference between the high and low per day. Some include the previous day's close in the TR calculation, e.g., the maximum between the previous day's close and the following day's high or low, whichever is greater.

It would be nice if the OP stated what he had in mind. It isn't obvious how to apply ATRs to options.These TR values are averaged over time, 14 days being the standard, yielding the ATR.
I personally use it as a factor in decision making expression, but not as a "system" per se.
Quote from Profitaker:
Any chance you could provide a worked example ?
Why not use a daily range probability given an assumed volatility in the underlying ?
e.g. Using a xx% vol, a 1 StDev range will be xx, 1.5 StDev range will be... and so on ?
