One way of getting about validating the data and calculations is to go to the definition of each variable - what they include and how they add, subtract, multiply or divide. Whether it is for a bank or industrial firms, a negative number for RoA is possible when the net income is negative. Asset values are either positive or zero, where a zero asset value with a positive net income is a fairly meaningless number. Technically, it is infinite when a positive number is divided by zero. In business terms, this is probably the "holy grail" (in jest of course) for managers where they earn something out of nothing.

I tend not to rely on database calculations when there are raw data that are available. Provided the raw data are presented in a consistent manner, I would prefer to show them and show how the results of the calculations are derived from the raw data.  

Thank you for your answer.

You can find in attachements data for a bank get from Bloomberg. Maybe you can help me to know how is ROA estimated.

Thank you in advance

As I suspected, the few that has positive net income (although small values) and asset (for banks, these are loans and properties or fixed assets essentially) gave a negative RoA. These are errors. Fortunately there are only a few.

If they disclose the ageing of the non-performing assets, this could be expensed if there is very little chance of getting it back (or they are unsecured by collaterals).

RoA in this case is simply dividing net income by the total asset figure.