I bet it is because of the noise...

His code, typo corrected, works.

If you put that code in a custom form method, say refreshList(), and call the method from list's When, Click and ProgramaticChange, the summ will always refresh.

Of course, he can check how to use grids where a list is just fine, all listbox samples, read 1000's lines of code and a book or two... I guess in a long, long run it is not even a bad thing. Anything can be solved given enough time and money, but you won't get enough time or money.