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This function returns x if x is not null. If it is null, it returns 0 instead. An undefined or empty field is "null." Null values don't work in numeric calculations, which is where the Nz() function comes in handy. When Nz() finds a null, it sees it as a zero. So, if you want to perform calculations on a field that may include a null, use the Nz function.
Nz(4) returns 4 Nz(0) returns 0 Nz(null) returns 0 Nz([Mon]) + Nz([Tues] + Nz([Wed]) + Nz([Thurs]) + Nz([Fri]) returns the total of all numbers found in these day of the week fields. If a field is empty, Nz() reads it as a zero.