IF formula evaluates to wrong value

Hi

I have the following formula: IF(SELL_INVESTMENT = "YES" , 1,2) It returns not 1 or 2 but 0. The worksheet contains over 1000 cells. Trace Precedents verifies that the formula evaluates the correct cell and that it contains "yes". I created a very simple worksheet to test the formula and here it correctly returns 1. I cant figure it out.
I am stuck.

To perform an IF one usually uses a cell or named cell to check on. What is SELL_INVESTMENT?

HI;

Thanks for your response. I found my problem, it turned out to be circular reference. Interestingly it turns out that the IF formula returns a zero instead of an error notice. Error checking for circular reference reveals the problem. The cell with the formula is not marked as an error. Only after saving and then reloading the spreadsheet is there a notice that there is a circular reference error. This can be confusing.Again thanks for your response. I hope that my experience can be of help to you.