The true range indicator is the greatest of the following:
-current high less the current low.
-the absolute value of the current high less the previous close.
-the absolute value of the current low less the previous close.
The average true range is a moving average (generally 14-days) of the true ranges. Simply put, a stock experiencing a high level of volatility will have a higher ATR, and a low volatility stock will have a lower ATR.
Nilesh Deshpande won the "Expiry contest" in "JustNifty" & explain the basis on which he had 'figure' it as follows:
"ATR works on most days, but not on all days.
My formula is simple.
PC-ATR*.80 to PC-ATR*0.50 Buy Zone
PC+ATR*0.50 to PC+ATR*0.80 Sell Zone
where PC is previous day's close.
Bears get weak near PC-ATR*0.50 to PC-ATR*0.75, but get charged up below PC-ATR*0.80 for a target of DH-1.5*ATR
Bulls get weak near PC+ATR*0.50 to PC+ATR*0.75, but get charged up above PC+ATR*0.80 for a target of DL+1.5ATR
where DH & DL are current days High & Low respectively.
Gives out better results when used with ADX for determining whether this is a trending or a sideways market."