A: Select the column that contains the nulls in the Select Variables dialog and then go to the Data Doctor tool located under the Validate menu.  This tool will display all of the “problem” data in a column such as nulls, 0’s, below detection limit, text entries etc and suggests replacement values that can be overwritten by custom values if desired.  The changes are also stored in a log file so you can see everything that has been done to a dataset since it has been imported into ioGAS.