When writing the Quickfingers Luc base scanner (Marvin) script, I wanted a measure of volatility that would be comparable between charts. The traditional Average True Range (ATR) indicator calculates a discrete number providing the average true range of that chart for a specified number of periods. The ATR is not comparable across different price charts.
Average True Range Percent (ATRP) measures the true range for the period, converts it to a percentage using the average of the period's range ((high + low) / 2) and then smooths the percentage. The ATRP provides a measure of volatility that is comparable between charts showing their relative volatility.
Enjoy.