Microsoft Office Reference Guide

Mar 26, 2004

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Sometimes I feel as though I don't get into Excel as deeply as I might, as the Office Guide. So I want to credit this week's update to an Excel Guru, Jeff Davis, who works extensively with Excel in a highly charged business environment.

Recently, we covered Excel charts which I use a lot in my work with presentations. This week, I am pleased to include a section on the nuts and bolts involved in creating the underlying spreadsheets, and some tips for keeping the calculations in Excel error-free.

When you invest a significant amount of time designing a spreadsheet to solve a business problem or generate a key business report, accuracy is "job #1." The last thing you need to worry about are fat-fingered keystrokes that wipe out entries or replace them with blanks. The best defense against errant keystrokes is knowing how to write formulas that identify "bad" entries and take corrective action before the errant results get published.

Perhaps the most common error to be trapped by spreadsheet gurus occurs when a cell absolutely, positively has to contain a number. That is, it can't be blank, and it can't contain a text entry. Otherwise, a formula referring to that cell will return an error message.

Fortunately there's ISNUMBER, the function that lets you test a cell's contents before you perform an operation on it. If you use ISNUMBER by itself, it takes the form ISNUMBER(CellReference). If the cell named by CellReference contains a value or an expression that returns a value, then ISNUMBER returns a logical value of True.

How do you use the ISNUMBER function to help identify problem cells in your worksheet? One way is to wrap an IF test around the ISNUMBER function to create an expression that displays the result of a calculation (if the cell contains a value) or a custom error message (if the cell is blank or contains a text entry).

To illustrate, suppose you want to multiply the value in cell A2 by the value in A1. If one of those cells contains a value and the other contains a text entry or is blank, then +A1*A2 will return the #VALUE! error message. However, you want to be the master of your error-message domain. To display something other than #VALUE! when A1 is blank or contains a text entry, try an expression like this:

=IF(ISNUMBER(A1),A1*A2,"Yo! Need a number here!")

Checking for Blanks

One of ISNUMBER's cousins from the "IS" family of functions, ISBLANK, also comes in handy when you want to display a custom error message. It takes the form =ISBLANK(CellReference) and evaluates to true only when the cell named by CellReference is blank. So, to bring attention to a cell that is blank (regardless of whether it's supposed to contain a text entry or a value), you can rewrite our first sample expression like this:

=IF(ISBLANK(A1),"Yo! Need a number here!",A1*A2)

The AVERAGE Function

The thing about AVERAGE is that it's a smart function. When you point it at a range of cells, this function knows to ignore blank cells when it calculates the average. The AVERAGE function returns a value, as long as at least one cell in the range contains a value or a number.

If you enter the expression =AVERAGE(A1:A10), and all those ten cells are either blank or contain text entries, AVERAGE will display the infamous #DIV/0 error message. Excel's help text about the AVERAGE function provides a simple and elegant example of how to display your own custom message instead of #DIV/0. In our example, averaging values in A1:A10, the expression looks like this:

=IF(ISERROR(AVERAGE(A1:A10)),"Yo! No numbers here!",AVERAGE(A1:A10))

In this case, if the AVERAGE function returns an error message, the ISERROR function evaluates to true, which tells the IF function to display the custom error message.

Summary

Understanding how empty cells and cells that contain text affect calculations is a core skill for all Excel users. To enhance your worksheet design, use the IF function in conjunction with the "IS" family of functions to display custom error messages.

InformIT Articles and Sample Chapters

Looking for a great reference for new Excel users who want to learn more about functions and how they work? Check out Functions: Fast Formulas for Fast by Sherry Kinkoph. This article includes a rudimentary definition of functions and arguments, tips for entering functions, and a table of "useful functions you won't want to live without."

Using functions to perform calculations on individual cells is just the tip of the worksheet design iceberg. To take your functions to the next level, you need to know how functions process data over ranges spanning two or more rows or columns. The "D" or database functions provide tools you can use to display summary based large amounts of data. To find out how they work, check out Excel 2000 Database Functions Overview by Patrick Blattner and Laura Stewart. This article takes an in-depth look at a baker's dozen of the most useful database functions.

Books and e-Books

Read Special Edition Using Microsoft Office Excel 2003 (Que, 2003, ISBN 0789729539) by Patrick Blattner, and you don't have to upgrade to Office 2003 to start putting his tips to work. Check out this sample chapter on Text Functions and learn an easy way to display numbers with suffixes such as st as in 1st, th as in 13th, and rd as in 33rd.

And once again thanks to Jeff Davis for making these tips available. He can be reached at his Web site, http://thejeffdavis.com.

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