The Delete command in the Edit menu designates the selected cells as having missing data (NULL). The Delete command is only available for cells containing data for variables that may be edited. To designate the value of a cell as missing data, use the Delete command. The Delete command sets the underlying data to NULL. Entering an empty string in a cell or actually typing the word “NULL” into the cell sets the underlying data to the string value “” or ”NULL”, respectively.
It is important to distinguish between deleted values and values set to the string “NULL”. Typing the word “NULL” sets the underlying data to the string value “NULL”. Similarly, copying a cell with no data to another cell does not delete the data in the other cell, but rather sets its value to the string value “NULL”. In order to determine whether a value has actually been deleted or is set to the string value “NULL”, view the file’s annotations. If the value was deleted, NULL will appear in the annotation without quotes. If the value was set to the string value “NULL" in quotes.