How should I represent missing values in my worksheet?

In general, you should just leave missing cells blank. You can also represent numeric missing data with a period. Missing data for strings can be represented by an empty string (entered with a single quote). However, blanks work just as well as any of these alternatives. If your worksheets already use a specific string to represent missing, you can tell Stat/Transfer to use that string as a missing value by setting the Numeric Missing Value string in Worksheet options.