A family of Microsoft relational database management systems designed for ease of use.
Sct80,
IIf functions are:
Condition to be tested: in your cae Number 4 = 0 and Test = Assay
Result if TRUE = "No"
Result if FALSE = "Yes"
So, you have things backwards.
See:
http://zo-d.com/blog/archives/programming/microsoft-project-tip-formulas-and-the-iif-statement.html