Definition:

The Z-Ratio calculates the difference in low frequency power (LF: 0.5-8Hz) to high frequency power (HF: 8-20Hz) in relation to the total available power.  The result is in a range from -1 to 1 where -1 indicates high frequency power concentration and 1 indicates low frequency power concentration.  

Calculation:

The equation that is used for this calculation is:

Z-ratio = (LF-HF)/(LF+HF)