Excel features yet another group of functions closely related to
the lookup and reference functions. Called *information
functions*, they let you retrieve information about the type
of content found in any particular cell you want to examine. You can
find the full list of information functions in the Information group
(choose Formulas→Function Library→More
Functions→Information).

The most important information functions are those that start with the word IS. These functions let you test whether a cell's blank, has numeric content, and so on.

The IS functions are Boolean functions, which means they give you a result of either TRUE or FALSE. On their own, the IS functions aren't too impressive. However, you can combine them with other conditional functions to make simple decisions. (The IF() function tests a condition, and then inserts one of two values based on whether the condition's true or false. For a refresher on the IF() function, see IF(): Building Conditional Formulas.)

You could use the IF() function in combination with the ISNUMBER() function to avoid performing a calculation if a cell doesn't contain numeric content. Here's the formula you'd use:

=IF(ISNUMBER(A1), 10/A1, NA())

In this example, if the ISNUMBER() test returns TRUE (that is,
if A1 contains a number), then Excel uses the first argument. That
means the program performs the calculation
*10/A1*, and displays the result in the cell. If cell A1 doesn't ...

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