Testing image location in document – this is a bitmap image

Function
Returns TRUE if
ISBLANK (Reference)	Reference refers to an empty cell.
ISERR (Reference)	Reference refers to any error Reference except #N/A.
ISERROR (Reference)	Reference refers to any error Reference (#N/A, #REFERENCE!, #REF!, #DIV/0!, #NUM!, #NAME?, or #NULL!).
ISLOGICAL (Reference)	Reference refers to a logical Reference.
ISNA (Reference)	Reference refers to the #N/A (Reference not available) error Reference.
ISNONTEXT (Reference)	Reference refers to any item that is not text. (Note that this function returns TRUE if Reference refers to a blank cell.)
ISNUMBER (Reference)	Reference refers to a number.
ISREF (Reference)	Reference refers to a reference.
ISTEXT (Reference)	Reference refers to text.

These functions are often used as the Logical_test in an IF function. For example, it's possible that the cell a formula will use as the divisor might be blank. The formula would then produce an error because the blank cell has value 0, and division by 0 is impossible. To prevent this from happening the formula can be included as one value of an IF, and one of the IS functions can test to see if it should be performed. For example a column called Average could be calculated as follows:

= IF (ISBLANK (Count), "", Total / Count)

This formula "looks" at the current value of Count to see if the cell is empty. If it is, the cell is left blank because "" represents the NULL string, i.e. a string with no characters. Only if the ISBLANK function returns FALSE will the calculation be attempted by Excel.

Of course it might happen that the contents of Count might not be a number. In such a case the ISBLANK function will report FALSE, but the text will evaluate to 0 and the error message will appear. So it might be better to use a different test:

= IF (ISNUMBER (Count), Total / Count, "")

Or if the number might be 0…

= IF (AND (ISNUMBER (Count), Count > 0), Total / Count, "")