The following table summarizes some of the the most common errors that you can make when entering a formula and how to correct those errors:

Make sure that you… More information
Match all open and close parentheses Make sure that all parentheses are part of a matching pair. When you create a formula, Microsoft Office Excel displays parentheses in color as they are entered.
Use a colon to indicate a range When you refer to a range of cells, use a colon (:) to separate the reference to the first cell in the range and the reference to the last cell in the range.
Enter all required arguments Some functions have required arguments . Also, make sure that you have not entered too many arguments.
Nest no more than 64 functions You can enter, or nest, no more than 64 levels of functions within a function.
Enclose other sheet names in single quotation marks If the formula refers to values or cells on other worksheets or workbooks and the name of the other workbook or worksheet contains a nonalphabetical character, you must enclose its name within single quotation marks ( ' ).
Include the path to external workbooks Make sure that each external reference contains a workbook name and the path to the workbook.
Enter numbers without formatting Do not format numbers as you enter them in formulas. For example, even if the value that you want to enter is $1,000, enter 1000 in the formula.