NAReturns the “value not available” error, `#N/A`.
#N/A is an error, so both the ISNA and ISERROR functions will return TRUE. Use ISERR to find errors which are not #N/A.
Typing =NA() into a cell is equivalent to directly entering the error value #N/A.
#N/A is used to mark missing information and to indicate to functions operating on ranges or cells containing such values to halt calculation. For instance, if cell B2 contained the result of an IF statement: =IF(ISBLANK(A1),0,A1) and B2 was subsequently involved in a sum or other formula, that formula would assume that B2 held the correct information. By altering the formula in B2 to =IF(ISBLANK(A1),NA(),A1), any subsequent operation on B2 would halt upon encountering the #N/A error, and return that error.
#N/A errors indicate missing information and signal functions to cease calculation. Use the #N/A value instead of 0 or the cell’s results. For example, if A1 contains the value #N/A or =NA(), the formula =A1+A2 will evaluate to #N/A.
ISNA: Checks whether a value is the error `#N/A`.
ISERROR: Checks whether a value is an error.
ISERR: Checks whether a value is an error other than `#N/A`.
Checks the validity of the data before further computing, in order to avoid mis-calculation.