Understanding Society User Support

I can see for many, individual monthly gross income is negative which is unusual.

Variable | Obs Mean Std. Dev. Min Max
a_fimngrs_dv | 50,994 1458.485 1592.191 -12495.48 15000
b_fimngrs_dv | 54,597 1540.385 1593.65 -15000 15000
c_fimngrs_dv | 49,739 1594.767 1597.395 -8000 15000

I have looked at the documentation, but not sure I have found the information. Any explanation for this?

Thanks, any response highly appreciated.

Ujjwal Kumar Das
University of Leeds