If you just write (IA_Tipo_de_Evento='Indisponibilidade'), the value of IA_Tipo_de_Evento may not be unambiguous in the context you are using the field in. But to answer that, you need to provide more information:
What is the context your using this expression in? A chart with dimensions, and if so, which dimensions? How does your data model look like?
Could you post some sample lines of data or a small sample QVW?