ATR is calculated using the highest of the following daily values: H - L; H - PDC; and, PDC - L.
For example, assume both H and L equal 13.19. The ATR formula would use the highest of these values: H - L = 13.19 - 13.19 = 0; H - PDC = 13.19 - 13.48 = -0.29; and, PDC - L = 13.48 - 13.19 = 0.29.
Those three components will always produce at least two values that are not less than 0; thus, ATR cannot be negative.
