  The stabilities of the beam and machine have almost the highest priority in a
modern light source. Although a lot of machine parameters could be used to
represent the beam quality, there lacks a single one that could indicate the
global information for the machine operators and accelerator physicists,
recently. A new parameter has been studied for the last few years as a beam
quality flag in Shanghai Synchrotron Radiation Facility (SSRF). Calculations,
simulations and detailed analysis of the real-time data from the storage ring
had been made and interesting results had confirmed its feasibility.
