CEQ "Common/Ordinary Equity" is composed of four elements, some of them could have a negative sign.
1. Common Stock, data85
2. Capital Surplus, data210
3. Retained Earnings, data36
4. Treasury stock adjustments, data88
In many instances, you may find companies that are having very negative Retained Earnings, which will drive the negative sign in Common Equity. Please refer to the Compustat Manual for more information.